425 mixer on chevy 350

[munkymush]

ive got an impco setup with a 425 mixer and a model e vaporiser on a chevy 350 in a ski boat .it runs fine apart from an intermittent fault where after a run the tickover sometimes stays up at around 900/950 revs .. it then will go down to normal tickover a minute or so maybe 2 later...
ive checked all linkages cable etc .. everything is ok ,free and allows the throttle flaps to always close correctly..checked for air leaks . is it worth me stripping the mixer and whats the likely outcome? the diaphragms bits etc look like they cost almost as much a another mixer , or am i looking in the wrong places for bits..would like a few views on wether I shud chuck it or repair it? thanks ..

[Gilbertd]

High tickover is usually down to an air leak. It could be something heat induced, check all vacuum hoses for iffy connections or splits. It's unlikely to be the mixer or vaporiser.

[munkymush]

ive replaced all the hoses .. ive sprayed around all joints while its doing it and cant detect anything, ..i will take it off and replace all the gaskets for base plate and all the joints .. its got a holly throttle plate under it.. wheres the best place for gaskets in the uk

[LPGC]

Gilbert pointed out you might have a vacuum leak, I'd agree that's the most likely cause, would check before messing with the LPG system. Boat forums have a lot of posts about spurious manifold vacuum leaks....

Since you've removed the petrol carb you can't check to see if the engine idles faster on petrol or LPG with the same idle stop position... You might have opened the idle stop to compensate for what is usually a lean mixture on LPG at idle, the high tickover might just be how it idles with a correct mixture - You might have got the problem / no problem situations mixed up, the high tickover might not be the fault, the usual normal tickover might be the fault condition but you compensated the idle screw for the usual lean mixture - If the mixture then became correct for some reason the idle speed would increase.

Other possible causes:
Could be the diaghpram in the mixer itself, there is one in the 425 mixer besides the one in the reducer.
Would usually expect a slightly richer mixture after a run because the LPG temp is usually lower when gas has been drawn at a higher rate from the reducer, colder gas is denser making for the richer mixture. This would backup the point in above paragraph.

Raw water cooled or closed cooling system? Have you felt the exterior temp of the reducer under various conditions?

Simon
Lpgc

[munkymush]

reducer is very hot all the time, no lack of heat, it does tickover lovely at 650 rpm for most of the time ..but it just lifts to 900/950 at times then settles down to 650 again ..its raw water cooled ..the temp gauge shows 160 degs consistantly which what its supposed to when ok.where is the best place to get gaskets for holly base plates which is whats under the mixer..

[LPGC]

where is the best place to get gaskets for holly base plates which is whats under the mixer..

Any American vehicle parts supplier (such as Rodley Motors) or Tinley Tech might be able to sell you one. Tinley Tech could sell you a rebuild kit for the mixer and reducer too.

I didn't necessarily mean the gasket under the mixer, manifold gasket problems and even cracked manifolds are apparently common on boat applications, at least according to boat forums.

Have you checked the distributor? Maybe it sometimes idles with too much timing advance due to sticking bob weights?

Simon
Lpgc