200 psi compression 50mm BBK

[motodude]

Hey guys so I have been dealing with something odd. I know these bigger bbks are known for failure but I installed one on my 139qmb as a kind of experiment. It ran fantastic all last year, but then this year it started hard cutting out at higher rpms. Did everything I could think of to remedy the issue (rejetted carb just about every size between 85 to 98 till the plug was a nice chocolate brown, replaced it with a carb that says “Keihin” on it, checked to make sure timing was on and valves were properly adjusted). Everything seems in order, but I did a compression test and hit 200psi… is that typical for these? This gauge also tends to read a bit low lol. Any thoughts on it are really appreciated, I’m just at a loss. Will probably be going back to a 47mm if I can’t get it sorted. Thanks!

[GrumpyUnk]

It could be the spark is getting a bit weak, and not firing under higher pressure. You could try a new plug, wire, or coil and see if that helps. It is harder for a plug to fire with higher compression.
tom

[motodude]

Jun 30, 2024 8:11:40 GMT -5 GrumpyUnk said:

It could be the spark is getting a bit weak, and not firing under higher pressure. You could try a new plug, wire, or coil and see if that helps. It is harder for a plug to fire with higher compression.
tom

Hey Tom! I just replaced the plug, but will look into trying a new coil and wire to see if it makes a difference. Thanks!

[pili]

I don't think I've ever read of someone measuring it that high, maybe small 4 strokes run higher. If you can get it sorted you might have a good runner there!

[90GTVert]

200 sounds high, but if it ran great for a year then I wouldn't be that worried about it just yet.

If you suspect a weak spark may be at fault, something that can help you figure it out is to gap the plug tighter. Run about half the gap or so. If you're doing say 0.028" now, try 0.014". That's not a fix and that's not where you should want to keep the gap, but if performance suddenly improves then it's likely that you have a weaker spark than it needs. You cna also try spark strength testers. Basically force it to jump a large gap and see if the spark is still consistent and bright. It doesn't create a really big gap, but even an old plug with the ground strap cut/broke off or even just bent way back can show you more than looking at the spark on a standard gapped plug.

amzn.to/3xD9wTG

[motodude]

Jun 30, 2024 23:05:02 GMT -5 90GTVert said:

200 sounds high, but if it ran great for a year then I wouldn't be that worried about it just yet.

If you suspect a weak spark may be at fault, something that can help you figure it out is to gap the plug tighter. Run about half the gap or so. If you're doing say 0.028" now, try 0.014". That's not a fix and that's not where you should want to keep the gap, but if performance suddenly improves then it's likely that you have a weaker spark than it needs. You cna also try spark strength testers. Basically force it to jump a large gap and see if the spark is still consistent and bright. It doesn't create a really big gap, but even an old plug with the ground strap cut/broke off or even just bent way back can show you more than looking at the spark on a standard gapped plug.

amzn.to/3xD9wTG

Interesting, I did not know that about gapping it tighter. Going to try this and see what happens when I get a chance! I looked into those “performance” cdi’s but they seem like a gimmick unless anyone can attest to them making a difference? Also would the iridium plug be better than the standard for this? 2 other things I realized I forgot to mention are that
1. it’s always had a little bit of a “pre ignition” tapping sound since I installed the kit
2. The cutting out issue gets exponentially worse as it gets warmed up
Thanks guys!

[90GTVert]

Don’t bother with a performance CDI… especially right now. A stock one would be better if you wanted to make sure that’s not failing.

Iridium won’t change much. I’ve had them make it easier with hard starting stuff, but there was always an underlying issue. The plug is not the cure. Best bet with the plug is make sure to use quality plugs in the proper style and heat range. NGK CR7HS is a popular choice. If there’s much detonation, you may find “peppering” on the porcelain of the plug. Not good. Detonation kills engines. At least use premium pump gas if not already doing so.

[GrumpyUnk]

I think there is a YT video showing the 'performance CDI' to be a scam. The little box is filled with sand, to make it seem to be more substantial. It is not. AFAIK, the performance CDI's do not work as well as the OEM and standard replacements available for much less.
The higher the plug gap, the higher the voltage necessary for the spark to jump the gap. When spark jumps in higher pressure mixture, apparently it is harder to do. I do not know the physics, but that's what I read.
I suppose it is possible that the compressed gas can 'blow out' the spark and cause misfire. I have a Kohler on a mower that will misfire a tiny bit when I run over things and the engine gets jarred. I do not have any proof at all, but suspect that the jolt causes the spark to fail to jump the gap properly causing it to not combust. I have not investigated any deeper as I am satisfied with my own explanation and it really causes no harm as far as I can tell.
I have used thicker head gaskets(composition with fire ring) when installing BBKs. I figure a tad lower CR is an ok thing to have when re-using the standard cylinder head. The 'BBK cylinder heads' have a shallow groove machined into the head surface around the diameter, thus opening up the area where the piston would get close to the original surface the width of the larger bore. A flat ring around the cylinder head the width of the 39mm to the BBK diameter. I cannot prove anything, but figure it is kind of like a 'quench' area.
tom

[motodude]

Okay that was my suspicion that they were just fakes marketed as performance parts, so good to know. I shortened the gap to .020 inches as that’s as low as my gauge goes, and right off the the bat it was idling at a higher rpm and almost rolled off the stand on me. As far as the cutting out issue, it was less frequent, but still there at full throttle. I can reliably replicate the issue if I am at full throttle and let off a little bit, and then go back to full throttle. Feels exactly like it’s losing spark for a little over a half a second. So this whole time I thought it was a carb thing but it definitely seems like a spark thing. Ordering the parts and will update when I have them all installed and tested. Really thankful for all the info from everybody! Learning a lot very quickly.

[motodude]

Also if I were to double up a gasket, would it be better to do the base gasket, or the head gasket? Head gasket just worries me a bit with leaks, and idk if moving the whole thing up with an extra base gasket will throw things out of whack? Or would I just be better off buying one of the aftermarket heads that come with the big bore kit and throwing that on? Thanks!

[GrumpyUnk]

If I were trying to cut the CR, adding base gaskets seems a lot more likely to be successful over the long term. The only pressure spot that would leak on a base gasket is the oil transfer slot(?) from the hole in the crankcase over to the cylinder head stud. Oil then travels around the diameter of the stud to the cylinder head and lubes the cam & cam followers. The pressure is on the order of a few psi at the highest as there is nothing except the length of travel and the height difference to build pressure. The outlet is not metered to keep pressure high.
The head gasket has to handle compression pressure and the heat of combustion. Stacking two gaskets would depend on the two sealing to the head & cylinder and also to each other.
There would be a slight change in the geometry of the cam chain. The cam sprocket would be a bit further away from the sprocket on the crankshaft. The cam and rocker/follower geometry would not change, the whole assembly being moved in tandem with the cylinder being moved.
I think I would consider getting some gasket paper that is thicker than the green paper base gaskets if my goal was to decrease peak compression. A single thicker sheet would eliminate any gasket-gasket leakage. The thicker paper should handle the low oil pressure without problem, and you could get a higher 'stack' than you likely would get with a pair of base gaskets.
The piston would effectively have a shorter stroke by the difference in gasket thickness. It would just not reach as high in the bore by xx thousandths. Not functionally much different, but a lower peak compression pressure developed.
tom

[motodude]

Alright I know it’s been a minute but I am back lol. I ended up going the route of buying the head that comes with the 50mm kits that has a dome cut out. I installed it and everything went smoothly and it brought the compression down to around 170 which seems much more reasonable. However starting it up it ran exactly how it did before and I was disappointed to say the least. I sat around messing around with things for a while seeing if anything made a difference including taking the air filter off. I always just popped the air filter off the little rubber piece that is clamped down with a hose clamp so I could just pop it back on, but this time I was trying everything even if it seemed stupid. So I pulled the little rubber piece off the intake of the carb and boom. Runs fantastic. This makes absolutely zero sense considering it ran so good for so long without this issue. Why it happened now, I’ll never understand. Ordered a filter without the little restriction ring in the middle this time. I guess PSA don’t be an idiot like me and buy a filter that blocks the ports on the front of the carb. Thanks everyone.