620 revving uncontrollably

[wildmaninid]

on my way home lastnight i passsed 2 slow cars in my 77 620 pickup (l20b, 5 spd, Hitachi carb all emissions crap still hooked up for now) as i gunned it to pass them and then came back into my lane, the engine acted like the throttle was stuck. I made it home and checked all of the obvious stuff.....throttle cable not stuck, removed dashpot, backed idle screw all the way off......and still she is running at about 4k rpm. I am wondering if maybe i blew out a carb or intake gasket? EGR problem maybe? Butterfly seems to be closed all the way, no obstruction or anything..... The truck has factory A/c and the electric pump went out last friday, but I put an exact replacement on it, and I don't think excess fuel pressure would cause this problem.......i could see how it would flood it but I am right now at a loss without further tear down.......anyone have any ideas?

 

Thanks, Bill

[Buzzbomb]

on my way home lastnight i passsed 2 slow cars in my 77 620 pickup (l20b, 5 spd, Hitachi carb all emissions crap still hooked up for now) as i gunned it to pass them and then came back into my lane, the engine acted like the throttle was stuck. I made it home and checked all of the obvious stuff.....throttle cable not stuck, removed dashpot, backed idle screw all the way off......and still she is running at about 4k rpm. I am wondering if maybe i blew out a carb or intake gasket? EGR problem maybe? Butterfly seems to be closed all the way, no obstruction or anything..... The truck has factory A/c and the electric pump went out last friday, but I put an exact replacement on it, and I don't think excess fuel pressure would cause this problem.......i could see how it would flood it but I am right now at a loss without further tear down.......anyone have any ideas?

 

Thanks, Bill

 

What about the SECONDARY carb butterfly? Nudge the rod that is attached to it to make sure it isn't stuck in the open position. I'd bet this is your problem, FWIW. I've had this happen, and I'm sure many others with stock carbs have too.

[datzenmike]

on my way home lastnight i passsed 2 slow cars in my 77 620 pickup (l20b, 5 spd, Hitachi carb all emissions crap still hooked up for now) as i gunned it to pass them and then came back into my lane, the engine acted like the throttle was stuck. I made it home and checked all of the obvious stuff.....throttle cable not stuck, removed dashpot, backed idle screw all the way off......and still she is running at about 4k rpm. I am wondering if maybe i blew out a carb or intake gasket? EGR problem maybe? Butterfly seems to be closed all the way, no obstruction or anything..... The truck has factory A/c and the electric pump went out last friday, but I put an exact replacement on it, and I don't think excess fuel pressure would cause this problem.......i could see how it would flood it but I am right now at a loss without further tear down.......anyone have any ideas?

 

Thanks, Bill

 

You did not blow a gasket... this would only let air in and it wouldn't idle or run poorly. ERG stuck open would do the same thing. Excess fuel pressure would possibly flood the carb but this would cause poor running and idle.

 

My bet is that the secondary butterfly has stuck partially open (from flooring it to pass) and the motor is sucking fuel/air through it and this is causing the high revs. Look down the secondary with a flashlight. Try pulling down on the vacuum diaphragm rod to seat it.

[wildmaninid]

Good call gentleman. Been with to many weber equipped datsuns to think about those old sticky vacuum secondaries........fixed it right up thanks!!

[datzenmike]

Bzzbomb, didn't see your post! Ha great minds think alike.

[Buzzbomb]

Bzzbomb, didn't see your post! Ha great minds think alike.

 

That's ok, Datzenmike :) This is one of the few obscure things I knew about Datsuns before I found Ratsun. I've had it happen a few times myself, and it sure isn't fun :P

 

The thing that I wonder about with this problem is what causes it, and how do you stop it from happening? Is it from the actual diaphragm getting hung up in the vacuum pod, or is the butterfly getting hung up in the baseplate somehow? Those are the only two things that are part of that system as far as I know, so it would have to be one or the other, or both..?

 

I've tried putting some spray w/straw lithium grease on the butterfly shaft from the outside of the carb, and I haven't had it happen in a while. I don't know if it's because this actually worked, or because I don't get into the secondary all that much.

[datzenmike]

That's ok, Datzenmike :) This is one of the few obscure things I knew about Datsuns before I found Ratsun. I've had it happen a few times myself, and it sure isn't fun :P

 

The thing that I wonder about with this problem is what causes it, and how do you stop it from happening? Is it from the actual diaphragm getting hung up in the vacuum pod, or is the butterfly getting hung up in the baseplate somehow? Those are the only two things that are part of that system as far as I know, so it would have to be one or the other, or both..?

 

I've tried putting some spray w/straw lithium grease on the butterfly shaft from the outside of the carb, and I haven't had it happen in a while. I don't know if it's because this actually worked, or because I don't get into the secondary all that much.

 

I would say you aren't driving it hard enough to use the secondary and keep it shiny.

[banzai510(hainz)]

man your guys are good!!!!!!!!!!!