Engine bogs under load

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Oct 25, 2009
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#1
This one has me stumped so far. This is a relatively new 66/80 engine and is not even broken in yet. The engine starts fine, runs well, idles nicely, revs well ....so far so good but when I get on to ride it acts as if the engine is way too small for the load and just won't go, eventually sputtering out. I can see no evidence of head gasket leakage, no evidence of spark plug leakage. I put a new carb on it yesterday and still acts the same way. My first thought was losing compression and maybe it is...at the piston rings. It was working well and this situation has developed slowly and recently. Tomorrow I will remove the muffler and try it again but seriously doubt that has any bearing on it. Has anyone else had this problem? If so, what was the problem? I should mention that the clutch engages and dis-engages normally, as does the centrifugal clutch. Hopefully, someone else has resolved this problem already and can help.
Thanks for any advice, help or encouragement guys.
Woody
 


AussieSteve

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#2
G'day Woody.
I guess that this could be a lot of things, but first - what colour is the plug?
(I know, I always ask that.)
Second, what's the compression pressure, if you have access to a compression gauge?
Also, what head do you have?
With a stock, flat head, you can expect about 115psi at normal cranking, (starting), speed.
Mine's 142psi, (RSE billet head), so a slant head should be somewhere in between the two.
(What makes you think you might be losing compression, incidentally.)
 
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#3
Hi Steve,
The plug is a NKGB6HS it is a dark gray...should be tan. But this engine has only a few miles on it so still running 16:1 mixture. As I said, it starts fine, idles fine, revs like it should, throttle response is good. Just put a bit of load on it and she acts as the engine were undersized for the load. I can't ride the bike anywhere to get the compression checked and can't haul it in my car. I have a friend who may be able to come here with his guage if he has one.
This a PowerKing slant head 66/80 and it has run the way it's supposed to in the past. The plug is gapped at.024. The new carb has four notches for the needle, it came from NT with the clip in the second from the top. A change of plugs, carbs and cdi didn't make a difference. Fuel is 87 oct and with regular two cycle oil (approved by PK they say either regular or synthetic)
I think it's losing compression because it runs so good until loaded, the sound changes as it bogs and dies. Almost sounds like an engine starting with the compression release open except mine is stopping. I'll be pulling the clutch tomorrow just to make sure it's not a problem but don't think it is.
I also think it may be running a bit rich because my other bike will act in a similar manner if I forget to open up the choke all the way. I will also check the air filter for any blockage and make sure the choke is working correctly.
I plan to add a shift kit to this bike soon but not until the engine is right again.
I'll let you know if the things I do make a difference and what the compression is if I get it checked.
Thanks!
Woody
 

Al.Fisherman

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#4
Before you do anything else and most likely waste your time...do a compression test. If you are lucky and have a hill of sorts, get bike uphill with tester installed and peddle down. Follow the above PSI figures.

When checking why a engine has a performance problem, a compression test is a MUST.

A blown head gasket can be heard, where as bad rings will force fuel mixture out of the carburetor.
 
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#5
Thanks Ron,
I haven't been able to reach my friend who may have the equipment and the know how to test. We don't have a hill anywhere nearby but I do have another way to spin the engine over several times easily. I need to put a dab of grease in the gears anyway so may as well check the clutch while I'm there.
I haven't seen any fuel coming from the carb. I had a small gas leak up on the shut off valve but fixed that. Engine is clean and dry. My first suspect was rings since the engine works fine with no load. I don't know what could have damaged it. The oil mixture has been correct, I haven't over revved it, and it hasn't been ridden hard at all. Maybe just a lemon.
I'll let you know what I or we find out when we do. Thanks for the good advice and information.
Woody
 
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#7
Ron, The only change I made was to replace the stock carb with a better one from Spooky, that was after this problem developed though, I also replaced the cdi because of a defective sparkplug boot on the stock one, I did the damage to the boot by pushing it onto the spark plug with out removing the screw on tip of the plug.Knew I was supposed to remove it, just plain forgot...had a senior moment. The cdi was changed after the bogging problem developed also. I put the stock carb that I removed from this engine on a separate engine, engine runs just fine with it. So it's not the carbs, it's not the cdi and I'm pretty sure the clutch is fine too. So the engine is getting spark, fuel, and air like it should..so what's that leave except compression loss? I suppose there could be some damaged part inside that only drags under load but I have no idea what that would be.
Woody
 
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#8
funny question are you turning the choke to the open posistion after it warms up? also tighten your head bolts alittle.. and then make sure your clutch is not slipping and is fully connected.. i had the clutch problem till i adjusted it correctly.. mail me if you need help how.. i did alot of work to make it right and Silent... http://motoredbikes.com/album.php?albumid=977
 
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#9
Hello Thine,
Yes, I am opening the choke after the engine warms. This engine has always taken longer to warm than others though and won't idle with choke open for quite some time, once warmed up it operates well except for the bogging down.
I have adjusted many clutches so I know how to do that. I'll be checking that out today although I'm sure that's not the problem. I checked the throttle cable movement compared to the other bikes, it's identical so the cable is working normally as I figured since the engine revs as it should. I have another bike that squeals like a pig when riding, the clutch is adjusted right, the gears are properly greased, it's just one of those bikes that will squeal no matter what. I suspect a poor fit in the gearing. Ironically, that engine is the easiest starting, smoothest running of all of them. I have 5. Three bikes finished and a fourth in the works, the fifth engine is a backup. It may go on the cruiser with the bogging problem til I figure out whats with that engine.
Thanks for the info and help!
Woody
 
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