Almost no compression on 78 maxi

Hi guys, this is the first engine I've ever worked on ( I have next to no knowledge on these things ) - learning though!

So my 78 maxi died while I was riding it last summer. Currently I have spark and it seems like I'm getting fuel since I can get the carb to overflow but seems like my compression is very low. I used a new compression tester and at first thought it was broken, but after a few kicks it read like 15psi. My brother said he could feel air with the thumb test though. Maybe it's noteworthy that when you kick it it doesn't make that chug chug sound they make when they're trying to start, just a whirring kind of sound.

I recently took the cylinders off and replaced the piston/rings, as well as the base, head, exhaust and carb fitting gaskets. Also put in a new spark plug and new carburetor (I didn't set the gap on the spark plug or really do anything to the carb besides install the throttle cable).

My theory right now is that my gaskets aren't installed well enough. I did my best to remove the old ones with carb cleaner and a razor blade but didn't sand it, just slapped the new ones on with really really slight bits of old material still on there.

I'm also not 100% confident that I rewired it correctly (when I took the throttle off I accidentally disconnected the two wires coming out of it, may have switched them up when I reconnected them), could that be the issue?

Any help is hugely appreciated I've been gradually working on this for a couple months now and just want to get it running before it gets too nice outside!

Thanks!

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Re: Almost no compression on 78 maxi

ALSO I stupidly went after the old gasket materials with a flathead at first before finding out about the razor method - I definitely made some scratches on the cylinder

Re: Almost no compression on 78 maxi

At this point I'm assuming I have some bad air leaks - but how do I find them if I can't get the engine to start?

Re: Almost no compression on 78 maxi

Did you check the gap on the rings? Make sure the gaps are staggered? If yes, I would suggest that there is a problem with the head gasket or head?

Re: Almost no compression on 78 maxi

Gouges in aluminum w screwdriver are no good gonna have to mill those against a flat surface. Look in the wiki for instructuons. Pictures help.

Re: Almost no compression on 78 maxi

Had to look up what ring gap is on the wiki - I'll make sure to check that. If everything's in order I guess I'll just get to sanding the cylinders, thanks!

Re: Almost no compression on 78 maxi

Thanks, maybe it was a little foolish to think I could get away with it haha, will work on it when I get a chance and post pictures

Re: Almost no compression on 78 maxi

roots to wings /

freds guide. service bulletins on myrons mopeds. sounds like your clutch cable may be loose too. take plug out and place your thumb in the hole and turn the bike over. if you can't hold your thumb on you have more than 15psi.

Re: Almost no compression on 78 maxi

Yeah, definitely check your gaskets/mating surfaces. Depending on how deep the gouges are, the repair becomes more or less complex and costly. Hopefully some sand paper, a flat surface, and maybe a new set of gaskets will sort out your problem.

Also, I'd invest in a cheap torque wrench so you can get those head bolts tightened properly. Don't sweat the compression tester too much, the cheap ones are hit or miss.

Re: Almost no compression on 78 maxi

Did you properly jet the new carb? Ya don't want to seize the engine..

Did you hold the throttle wide open whilst testing compression?

Re: Almost no compression on 78 maxi

> roots to wings Wrote:

> -------------------------------------------------------

> freds guide. service bulletins on myrons mopeds. sounds like your

> clutch cable may be loose too. take plug out and place your thumb in

> the hole and turn the bike over. if you can't hold your thumb on you

> have more than 15psi.

I think my clutch cable is fine, it pulls the thing its attached to all the way (clutch lever?)

Thanks, I've done some reading on those guides now and I can see why people point to them on almost every thread on here xD

Re: Almost no compression on 78 maxi

> Aaron Blair Wrote:

> -------------------------------------------------------

> Did you properly jet the new carb? Ya don't want to seize the engine..

I didn't do anything to the carb, just put it on. I believe it came with an 82 jet, I still don't fully understand how to set a jet so will need to do some research.

>

> Did you hold the throttle wide open whilst testing compression?

Yes, I held the throttle open, seems like it pulls the thing (carb slide?) as it should.

Re: Almost no compression on 78 maxi

About to start sanding the head and cylinder, I know fred's guide says to use a file first but I don't have one on hand and currently can't handle another trip to the hardware store - saw other threads where people didn't mention filing so I'm hoping it'll be fine.

I noticed some black stuff on the head gasket which I think I read indicates air leaks.

I'm more concerned about the surface between the engine and cylinder but don't really understand how I'm supposed to sand it evenly - the cylinder has that extra lip of aluminum that inserts into the engine so how can I sand around that?

Also, in order to sand the surface on the engine I have to take those cylinder mounting studs/pegs out right? I'm assuming you just unscrew them but they're not budging and I'm afraid of snapping them off.

Thanks for all the help so far you all are amazing!

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Re: Almost no compression on 78 maxi

Update: I sat down to sand the head with 180 grit, then 220, 320, and then 400. I did a couple figure 8 passes with the 180 and instantly it was clear that it was adding tons of scratches to the surface. I thought this step was supposed to REMOVE scratches so now I'm very worried... maybe I shouldn't have skipped that filing step.

Should I keep going?

Re: Almost no compression on 78 maxi

That’s just how sandpaper works — you remove a lot of material (through scratches) with the larger grit stuff, then less and less with successively finer grits, each step smoothing out the last one. If you think you can get the screwdriver marks out with a finer grit, then you can start there. If that’s taking too long, just go back to a rougher one.

Re: Almost no compression on 78 maxi

> Chinatown Kicks Wrote:

> -------------------------------------------------------

> That’s just how sandpaper works — you remove a lot of material (through

> scratches) with the larger grit stuff, then less and less with

> successively finer grits, each step smoothing out the last one. If you

> think you can get the screwdriver marks out with a finer grit, then you

> can start there. If that’s taking too long, just go back to a rougher

> one.

Thanks! Honestly I thought a little harder about how sandpaper works after posting that and realized it was kinda a silly question.

I used 180 grit on the cylinder and head mating surface and then 220 for a little. Starting to get bored so I'm gonna put it back together and see if my compression tester shows any improvement, will update later!

Re: Almost no compression on 78 maxi

Zero improvement. I just wish I knew where the leak is :/

Any way to troubleshoot that when the engine can't run? I've tried listening/feeling for air but wasn't able to while kicking since I was alone. I'll post a picture of the mating surface later this week - would scratches no deeper than a hair really be to blame?

Ordered a feeler gauge to set the ring gap because I won't have time to go to the store until Friday anyway. Could that be the cause of complete lack of compression? I want to say no.

I plan on doing whatever it is that I need to do for the jet but that shouldn't effect compression, right?

Re: Almost no compression on 78 maxi

roots to wings /

you have a mismatching head and cylinder. you have a hi torque cylinder and a lo torque aluminum head. hi torque cylinders don't typically run head gaskets. I'd use a little permatex red on your gaskets and I'd get a hi torque head and a can of carb spray so you can see where the leak is.

Re: Almost no compression on 78 maxi

roots to wings /

Re: Almost no compression on 78 maxi

> roots to wings Wrote:

> -------------------------------------------------------

> you have a mismatching head and cylinder. you have a hi torque cylinder

> and a lo torque aluminum head. hi torque cylinders don't typically run

> head gaskets. I'd use a little permatex red on your gaskets and I'd get

> a hi torque head and a can of carb spray so you can see where the leak

> is.

Oh boy... It was running for a couple months after I bought it with that cylinder and head so that's interesting, I guess they're semi-compatible? lol. Thanks for the links I'll see about ordering one of those clean ones, I wonder though how can you tell the difference between a high torque and low torque? Does... does that mean the torque specs would be different?

Very interesting...

Re: Almost no compression on 78 maxi

roots to wings /

Re: Almost no compression on 78 maxi

> Aidan Schmidt Wrote:

>It was running for a couple months after I bought it with that

> cylinder and head

If the bike was running before with that head and cylinder, then I wouldn't add another variable by changing it up now. There are tons of mismatched OEM combos ripping around out there, not to mention all the hi hi comp heads that got slapped on bikes four days after their new owners placed their first orders at treatland.tv. More important right now are flat surfaces and consistent torque to spec on the mating surfaces (use a figure eight pattern on the head and go back and forth on the intake and exhaust).

The new carb may be interfering with your problem solving. If the bike was previously tuned for a 12mm carb and it now has a 15mm (with a jet about 20 sizes up from the previous one), then it's basically throwing the bike's respiratory system out of whack and making it even harder to start. Until you can get the bike running again, I'd use the old one and (again) eliminate that variable.

I hate to ask this because it's so obvious -- are you sure the new piston and rings are the right size for that cylinder?

You mentioned that the clutch arm responds to a pull on the lever, but can you tell if it's actually engaging? Like if you pull the clutch lever and kick over the engine, does the flywheel spin?

Re: Almost no compression on 78 maxi

> Chinatown Kicks Wrote:

> -------------------------------------------------------

> > Aidan Schmidt Wrote:

>

> >It was running for a couple months after I bought it with that

>

> > cylinder and head

>

> If the bike was running before with that head and cylinder, then I

> wouldn't add another variable by changing it up now. There are tons of

> mismatched OEM combos ripping around out there, not to mention all the

> hi hi comp heads that got slapped on bikes four days after their new

> owners placed their first orders at treatland.tv. More important right

> now are flat surfaces and consistent torque to spec on the mating

> surfaces (use a figure eight pattern on the head and go back and forth

> on the intake and exhaust).

>

Okay, I'll continue to sand - Kind of concerned with sanding the intake and exhaust evenly since the mounting peg things are kinda in the way though...

> The new carb may be interfering with your problem solving. If the bike

> was previously tuned for a 12mm carb and it now has a 15mm (with a jet

> about 20 sizes up from the previous one), then it's basically throwing

> the bike's respiratory system out of whack and making it even harder to

> start. Until you can get the bike running again, I'd use the old one

> and (again) eliminate that variable.

Both the old and new carbs are 12mm but I'm not sure about the jets - I'll try putting the old one back on

>

> I hate to ask this because it's so obvious -- are you sure the new

> piston and rings are the right size for that cylinder?

I forget how I determined the size, I think my old one had a number on it? I'll check when I get a chance. The piston fits in very snugly to the cylinder and the ring came with it, though I haven't set the ring gap yet.

>

> You mentioned that the clutch arm responds to a pull on the lever, but

> can you tell if it's actually engaging? Like if you pull the clutch

> lever and kick over the engine, does the flywheel spin?

I'll double check that when I get a chance, haven't made sure the flywheel spins but kicking it is definitely harder with the clutch so I assume it's working.

Thanks so much for the input!

Re: Almost no compression on 78 maxi

Sorry, I should have been more clear -- You want those surfaces to be flat (and a little light sanding on 320 grit will show you if they are), but I meant to go in a figure-eight pattern when tightening down the bolts at the cylinder head (like top left, then bottom right, then top right, then bottom left), a little bit at a time until you hit 7 foot pounds. For the intake and exhaust, tighten a little on the left, then a little on the right, again and again until you're at spec. This will ensure the torque is consistent across the face of whatever you're tightening, rather than really tight in one area and kind of loose (and prone to air leaks) in another.

You mentioned earlier that the jet in your new carb was an 82, which comes in the 15mm Bings. Is it possible that the new jet is a 62 rather than an 82? That sounds a lot more appropriate to a stock carb, airbox, and pipe (which we can't see from your pic).

Re: Almost no compression on 78 maxi

roots to wings /

Chinatown may be technically correct on the mismatching head, but if you think you are leaking I'd be doing everything possible to get a good seal. With stock hi tork head and cylinder you don't need a gasket to get a good seal. The lo tork metal gaskets need to be in very good condition to do their thing and I don't know how solid that seal is when mismatched. It might be fine, but it ain't spec. Definitely put the old carb back on until you get it running. Re-reading your original post tho; did you ever get the bike to turn over (chug) in any way after putting all the stuff on it? Was it ever running after your changes? Your clutch cable should be kind of taut along the belly of the tank and it ain't gonna start unless you can get it to chug. The clutch make a quick "vrrrrr r" noise when you try to start the bike if you pedal too hard and/or the clutch cable isn't adjusted properly and you'll feel like you are going to fall on the bike. There are nuts to adjust where the inner clutch cable goes into the outer cable housing along the bottom of the belly of the tank. Your leak would have to be catastrophic to get no chug. You can also hear a slight chug or blurp by spinning the flywheel with the head off. In fact, I'd check that first and before anything else. If you can't get it to do that with the head off and a few drips of oil in the cylinder, I'd take the cylinder off and inspect the rings and measure the ring gap (which I didn't understand how to do when I did it for the first time). I'd re-do the thumb test and post video of what you are doing and what you have once you get your new gaskets back on. Once you get chug, turn the engine to "on", take the plug out and test for spark.

Re: Almost no compression on 78 maxi

Jesse Nordlander /

Used copperkote spray on head mating surfaces without gasket. First pull the cylinder and see if rings are stuck. Worst case look into exhaust port and see if piston looks good.

Re: Almost no compression on 78 maxi

Looks looks like someone messed with the cylinder ports. Hard to tell by the picts but is there there chrome peeling? Whats the piston and rings look like, clean or scuffed?

Once the big scratches are out i usually end up with a few passes on 400 usually does it.

Re: Almost no compression on 78 maxi

If your seriously worried about it being a vacuum leak the next best way to find it without it running is called a "leak down test" look in the wiki and find out what you need just be carefull bc if you put to much air in it will blow out the crank seals I think the recommended is like 5 psi gl

Re: Almost no compression on 78 maxi

Pushrod Fifty /

I put my head on upside down once and the decomp hole didn't get covered but I see yours doesnt have the hole for the deleted decomp. Maybe your piston is upside down or your nuts are bottoming out onto the unthreaded part of the studs because of the different head. Or you dont have flat washers under the nuts and they are falling on the aluminum giving you a false torque reading , or you didn't use a torque wrench at 85 in-lb in a cross wise fashion. Any of these silly reasons will give low comp.

Re: Almost no compression on 78 maxi

Thanks for the input everybody and sorry for the lack of updates, I've been really busy. I haven't gotten a chance to look into everything you guys have suggested but I very much appreciate the info and am glad it's here for when I need it, but I had about an hour today to mess with it and I no longer think compression is my issue, but rather the clutch.

I turned the flywheel by hand with the compression tester on (which I originally thought might be broken) and it registered! The longer I turned the wheel the harder it got to turn and after a few turns the tester read ~45psi.

I guess where the real problem lies in the clutch, since when I kick it over with the clutch open the flywheel either doesn't move at all, moves a few mm, or turns like halfway around (very rarely, maybe once in 20 kicks). I don't fully understand what the clutch does or what the parts inside look like but my dad's short explanation was that with too much force the clutch slips off and thus the pedals don't interact with the engine. If you guys have any videos or readings that might explain common issues with the clutch I'd really appreciate it.

I also stuck a paper towel into the spark plug hole and kicked to see if it got wet with fuel and it didn't, so maybe I'm not getting fuel into my engine after all, not sure why that would be though.

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