Predator 212 dies at hot idle

[Luke5589]

Have a hemi predator 212 I’ve been building. Chinese mikuni vm22, header pipe with a screw on muffler (Amazon special), billet rod, pvl flywheel, and 18# springs. Main jet on the carb is a 110 and I’m at about 450 foot elevation, pretty low.

Engine starts fine with 2-3 pulls without the choke even when cold. Will idle fine while cold. However, once the engine warms up, particularly after riding for 10-15 minutes, it has trouble idling. If I let it sit for about 20-30 seconds idling it’ll just die.

Any ideas? I’ll attach a pic of the spark plug. Plug is basically brand new and has about an hour or two on it. I’m not an expert at reading spark plugs and images and charts on google vary widely.

Any and all help is very appreciated! Thank you.

 

[BrownStainRacing]

Have a hemi predator 212 I’ve been building. Chinese mikuni vm22, header pipe with a screw on muffler (Amazon special), billet rod, pvl flywheel, and 18# springs. Main jet on the carb is a 110 and I’m at about 450 foot elevation, pretty low.

Engine starts fine with 2-3 pulls without the choke even when cold. Will idle fine while cold. However, once the engine warms up, particularly after riding for 10-15 minutes, it has trouble idling. If I let it sit for about 20-30 seconds idling it’ll just die.

Any ideas? I’ll attach a pic of the spark plug. Plug is basically brand new and has about an hour or two on it. I’m not an expert at reading spark plugs and images and charts on google vary widely.

Any and all help is very appreciated! Thank you.

It's definitely lean on the high side. Needs more main jet.

Try tuning the low speed mixture screw when engine gets up to temp. Tuning while cold does nothing.

Double check your head bolts, make sure they are torqued to spec. These engines will start and run with a leaky head gasket.

You'll pull your hair out trying to tune the carb with a bad head gasket.

Stock thick head gaskets are more forgiving then the thinner shim gaskets. Stock can usally get away with jus tightening the bolts, or studs. Shim gaskets should be replaced and torqued to spec, checked after the 1st few hot, cold cycles til they stay torqued.

Might as well set valve lash to .003", while you are checking head bolts.

 

[Denny]

Set idle speed only when hot!

 

[Luke5589]

It's definitely lean on the high side. Needs more main jet.

Try tuning the low speed mixture screw when engine gets up to temp. Tuning while cold does nothing.

Double check your head bolts, make sure they are torqued to spec. These engines will start and run with a leaky head gasket.

You'll pull your hair out trying to tune the carb with a bad head gasket.

Stock thick head gaskets are more forgiving then the thinner shim gaskets. Stock can usally get away with jus tightening the bolts, or studs. Shim gaskets should be replaced and torqued to spec, checked after the 1st few hot, cold cycles til they stay torqued.

Might as well set valve lash to .003", while you are checking head bolts.

Thanks for all the info! I figured it needed a bigger main jet. Are you assuming this based off the spark plug picture or is a 110 jet just too small for these engines?

Also with the idle screw it seems to already be running pretty high rpm at idle, just bogs down slowly til it dies. I’ll do adjustments while hot. I wasn’t aware it needed to be adjusted hot, not cold.

I’ll check the head bolts as well. Currently a stock head gasket, however I’ve removed the head once or twice without replacing the head gasket. Probably was a silly mistake on my part. I’ll order a new one.

Valve lash was set to .003 last time I took the valve cover off.

 

[BrownStainRacing]

Thanks for all the info! I figured it needed a bigger main jet. Are you assuming this based off the spark plug picture or is a 110 jet just too small for these engines?

Also with the idle screw it seems to already be running pretty high rpm at idle, just bogs down slowly til it dies. I’ll do adjustments while hot. I wasn’t aware it needed to be adjusted hot, not cold.

I’ll check the head bolts as well. Currently a stock head gasket, however I’ve removed the head once or twice without replacing the head gasket. Probably was a silly mistake on my part. I’ll order a new one.

Valve lash was set to .003 last time I took the valve cover off.

Yes, reading the plug is telling me it is way too lean on the high side.

See the "like new" white porcelain??? That needs to be a tan color. A bigger main jet, high speed, will start turning the porcelain tan, or brown.

Try tuning it with the chain off or the tires off the ground 1st.

Turn idle speed up so it gets up to temp.
Slowly back idle speed down til it stays running by its self.

Adjust low speed mixture, 1/8-1/4 turn at at time, giving it a few seconds to catch up with its self for each adjustment. You will hear up and down unstable idle, look and listen for a nice steady beat.

You may have to go back to the idle speed adjustment to find that low steady rpm idle.

 

[Luke5589]

Yes, reading the plug is telling me it is way too lean on the high side.

See the "like new" white porcelain??? That needs to be a tan color. A bigger main jet, high speed, will start turning the porcelain tan, or brown.

Try tuning it with the chain off or the tires off the ground 1st.

Turn idle speed up so it gets up to temp.
Slowly back idle speed down til it stays running by its self.

Adjust low speed mixture, 1/8-1/4 turn at at time, giving it a few seconds to catch up with its self for each adjustment. You will hear up and down unstable idle, look and listen for a nice steady beat.

You may have to go back to the idle speed adjustment to find that low steady rpm idle.

Again thank you for all the info.

Another thing I completely forgot to mention, is after the engine is at operating temp and I shut it off, I can not start it without twisting the throttle. When it’s cold I don’t need to touch the throttle. Hot, needs some throttle twist to start. Does this also point to idle screw being calibrated incorrectly? What about my pilot jet? Could the pilot jet be too small as well?

Sorry for the million questions. This is all new to me and I’m still learning.

 

[BaconBitRacing]

You know, I have a lawnmower with a similar issue. no start when hot, though, keeps running till you turn it off then won't restart. I haven't gotten around to troubleshooting it yet.
Luke, keep asking questions, it's the best way to learn without breaking things. I'm definitely staying tuned to this thread.

 

[Luke5589]

Update! Was running lean. Fixed it by tuning air/fuel screw and idle screw while engine was hot. Also threw a 135 jet in. Might try a 140 later.

Thank you all for the help.