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Author Topic:   starts good, goes bad
merlinmech2
Dirt Maniac

Total posts: 131
posted April 17, 2005 08:27 PM  
9:1 355 flattops, 76cc heads,5.40 lift cam, pump gas, 36 timing. Runs good for the first 2 or 3 laps, but then starts missing at the end of the straights, After about 5 laps misses all the time. Ignition and carb were off another motor that ran good. Will turn 6500 the first couple of laps, then continues to fall. After it cools off,(only ran 200 in the race) it runs good again. Any ideas where to start looking?

racer2
Dirt Freak

Total posts: 352
posted April 18, 2005 05:46 AM  
if its a hei,id start with a coil and the pickup coil under the weights

TEAMPCR
Dirt Freak

Total posts: 238
posted April 18, 2005 06:16 AM  
Could be a module heating up and failing, also.

avila
Dirt Freak

Total posts: 217
posted April 18, 2005 09:39 AM  
what exactly is the mis like? does motor
start popping, stumbling or what?

merlinmech2
Dirt Maniac

Total posts: 131
posted April 18, 2005 05:36 PM  
Its not a steady miss. no backfire, popping or noise. can just feel a little vibration and loss of power. Swapped out dist.(hei) and carb, no change. Changed motors last nite. Will try to tear it down this week. Just don't know where to start looking.

Pickel
Dirt Forum Champ
Total posts: 784
posted April 18, 2005 07:51 PM  
hmmmm, sounds like it could be related to something as it heats up.

what about spark plug wires, losing
resistance as they build heat? valve lash becoming a little loose as motor warms up?

racer17j
Dirt Forum Champ
Total posts: 5390
posted April 18, 2005 08:36 PM  
What spark plugs are you running? Could be that you have to hot of plug. I'd reccomend the AC 43 or a similar heat range of another brand.

merlinmech2
Dirt Maniac

Total posts: 131
posted April 19, 2005 05:28 PM  
Put the new motor in today. Runs like a champ. Used the same ignition and carb. Havent had time to tear it apart yet. Will let all know if I find anything interesting...

El Santo
Dirt Full Roller

Total posts: 36
posted April 19, 2005 11:01 PM  
I had a similar problem with one engine a few years ago. It would run great for 5 laps and then it started to get real lazy and wouldn't turn any rpm's. Tore it down and found the piston rings were too tight. If you are running hypereutectic pistons make sure your ring gaps are wide enough and not touching when the engine heats up. I was lucky to find this early before I broke a piston. I widened the gaps and I still have that engine 4 years later.

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