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Discussion Starter · #1 ·
Help!
I have a 1974 blazer k5 rock crawler on 40" tires with a Chevy 350 engine on a propane fuel system. The fuel system consists of two fork lift tanks, a vff30 fuel lockoff Valve, than a ohg model x-1 pressure regulator/ vaporizer, and finally an impco model 425 mixer on top of a holly throttle plate.

I have a new MSD street fire ignition distributer and wires. I put in 1 heat range hotter plugs for propane and have nice blue spark. I have excellent compression. I have taken the vacuum hose off of the fuel lockoff VFF30 and sucked on it, this does open the valve.

I pulled all the plugs and I put gasoline in every spark plug hole. Started right up and ran excellent and then died and will not start. Fuel is getting past the fuel lock off to the regulator/vaporizer (R/V) because I can push the primer button and dump propane from the R/V into the mixer. Than I have to crank the engine again to create vacuum to allow the fuel lockoff to let more fuel go to the R/V prompt again. Somehow fuel isn't getting past the mixer? I even tried dumping gas down the mixer and this does not work. It's a impco 425 sitting on a holly throttle plate. I can smell propane getting to the mixer while the engine is turning over. It must be coming out the top of the air cleaner but somehow it is not getting past the mixer diaphragm into the engine ?

Help I'm a nooby to propane!


Andrew S. Johnson, P.E.
Civil Engineer
 

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Open up the fuel lockoff as thats also a fuel filter. Give that a check first. They can get kinda gunked up.

Have you had any backfires? One good backfire can tear the mixer diaphragm on the impco units and they wont work any more. A quick teardown of the unit could confirm this.
Same with the vaporizer. Open up the top and check the condition of its components as well.

It honestly sounds to me like your not getting enough vapor to the mixer. But its really hard to tell without being able to lay hands on the components.
 

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Discussion Starter · #4 ·
I had backfires. Looks like the diaphragm on the impco 425 mixer at the idle mix screw cover is torn and totally shot. I'm new to propane but I'm confident this is why the engine won't start on propane. I've made sure the fuel lock off and regulator are functioning by testing both with the hose to the mixer disconnected and putting a vacuum to the lock off and the regulator. Those components work perfect. The engine will run on gasoline dumped down the throttle plate without the mixer in place.

So the idle screw diaphragm isn't allowing fuel past the mixer, because it's torn. I can blow no pressure with my mouth through the fuel intake on the mixer, unless I lift up the main diaphragm with my hands. The diaphragm for the idle mix is shut and won't let any fuel past to start the engine! I'm willing to bet that if I lifted the main diaphragm while cranking the motor over it would fire up, but this maybe dangourous so I'll just get new idle diaphragm and replace the torn one.
 

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It sounds like there could be a vacuum leak causing this starting issue. More info is needed to diagnose the problem. How old are these components? Has the engine been running ok and the problem just started? You said you could smell propane when spinning the engine over, if so, where do you smell it? Generally if you can smell propane at any time there is a fuel leak or the engine is set up very rich. Have you removed the top of the vaporizer to verify the condition inside it? When you do look at the spring under the secondary lever and see what color it is. There are two different springs used, one for an OHG mixer and the other for an Impco mixer.

If you have problems figuring it out you can give me a call. I will be glad to help.
Sincerely,

Buddy Gamel
Precision Sales & Service, Inc.
451 64th Place South
Birmingham, Ala. 35212



877-403-7827, office
205-837-8871, mobile
205-591-2267, fax


[email protected]
[email protected]
Precision Sales and Service, Inc.;Birmingham, Alabama
WorkHorseSupply
 

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Discussion Starter · #7 ·
The diaphragm on the idle cover plate of the Impco 425 was completely distorted from old age and a few backfires from replacement of the distributor and the timing being off slightly. I ordered the idle diaphragm part and think this will solve the problem of not starting.
 

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Discussion Starter · #9 ·
The idle diaphragm on the Impco 425 came in the mail last night, I installed it and the engine fired right up and runs great. Now I know how to trouble shoot a propane system. I set the timing and tuned the idle and high speed mixer screws by ear and test driving.
 

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Below are the general directions for setting the timing. As far as the mixture settings the idle mixture is normally the only one that needs to be adjusted. The power mixture is preset to the proper mixture when the mixer was built. It is difficult to adjust the power mixture without an exhaust gas analyzer or some way to install an oxygen sensor and take a reading. If the power mixture is place about halfway in the middle of the opening in the bottom of the mixer it will be close.

Setting Timing for a Propane Engine



On most applications, the timing should be a total of 30 degrees(base and centrifugal) with all of it in by 2500 rpm. What we normally do is disconnect the vacuum advance, run the engine up to 2500 rpm, set the timing at 30, lock it down, then let it idle with the vacuum advance disconnected. See what your base timing is running. If it is low,4-10 degrees for example, you can alter the centrifugal to lower that number and increase the base, still keeping a total of 30 degrees. As you increase the base timing check to see if the engine spins and starts smoothly when hot. If you reach a point that the engine bucks or loads the starter, back off about 3-4 degrees and that is your base timing. Subtract that number from 30 degrees and that will be the advance you need to have. This will provide a good start, strong idle and proper advance curve. We have had some engines run as much as 20 degrees base timing but 14-16 degrees is normal. The vacuum advance should be connected to ported vacuum and have no more than 10 degrees. This helps fuel economy at light throttle positions.
 

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Discussion Starter · #12 ·
Thanks,
I have set the timing advance to 30 deg, runs great but propane has an unuasal strong smell from the exaust. Does that mean I'm running rich? I did take the plugs up one heat range from the usual unleaded gas heat range, because the trucks plugs where really black and sticky with tar-ish stuff when I pulled them after buying this rig from the original owner.

Also, can regular BBQ places like UHAUL fill my fork lift tanks, or do I have to take them to a special location?

Glad my photos finally showed up in this post, they were not working before.
 

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Propane smell is somewhat normal but it's hard to describe what would be excessive. I fill my tanks at the hardware store. The more I buy the cheaper it is. Bbq tanks are flat rate here but over 10 gallons its by the gallon.
 

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Thanks,
I have set the timing advance to 30 deg, runs great but propane has an unuasal strong smell from the exaust. Does that mean I'm running rich? I did take the plugs up one heat range from the usual unleaded gas heat range, because the trucks plugs where really black and sticky with tar-ish stuff when I pulled them after buying this rig from the original owner.

Also, can regular BBQ places like UHAUL fill my fork lift tanks, or do I have to take them to a special location?

Glad my photos finally showed up in this post, they were not working before.
If you can smell propane coming out the exhaust then the engine is running very rich. If yours is a stock engine, meaning no bigger cam with excessive duration, then you should be able to start the truck in a garage with the doors closed and it not smell bad. There are millions of forklifts running propane in warehouses around the world that operate all day long with the doors closed. Propane runs that clean when properly set up. Even if it is slightly off you should not be able to smell it. It sounds like this rig has had issues for a long time from being out of tune.

Any place that sells propane can fill your tanks if they the proper fill nozzle or adapter. U-Haul is not known for having the best pricing. You tanks should be filled and measured by the gallon and not the pound like BBQ cylinders.
 

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similar issue

Hey, this may have given us the answer to our issue also. We had our Impco setup running on a SBC 350 that blew, and just finished setting it up on our 4.3L V6. We have checked vacuum, fuel hoses, have strong spark at the spark plugs (we used the plugs from the 350 that were working fine), we have fuel when priming, but like you, can't guarantee it is actually entering the mixer like it should. We haven't gone through the parts to check diaphragm etc so we will do that. We have tried the timing all the way retarted and all the way advanced, 1 off, and 180. Tried all 4 possible firing patterns and have literally zero response when cranking. We at at a dead end, until seeing this about the internal parts.

Any other advice guys?

P.s. mr engineer, where did you order that idle screw diaphragm??
 

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We have Impco parts in stock if you need them. Anytime there is a backfire it is a good idea to check all diaphragms in the mixer and secondary side of the vaporizer. Generally the idle diaphragm takes a hit first as it is the most delicate. If the timing is anywhere close to top dead center it should fire off if there is a vacuum signal to open the vaporizer.

An easy way to tell if you have a vacuum leak with a 425 mixer is to spin the engine over with the air filter off and watch the air valve assembly. If it pulses up and down while the engine is spinning then there probably is not a vacuum leak. But if it does not move there is definitely a vacuum problem or the mixer has been warped. It is possible to over torque mounting screws/bolts and warp the housing or base plate. Make sure when installing on an engine that the air valve moves freely in the housing after it is bolted on.
 

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We discovered the air valve is not opening yesterday. Went through everything, and everything is intact at least. So he attempted to not fry his hand hairs off and sprayed ether down where it wasn't entering, but there was no firing so now our timing is way off. Tried 1 more time, but battery died. We've just been murdering the battery trying to fix this issue, even with a deep cycle it is starting to struggle. We haven't gotten it running yet at all on the 4.3L but it ran perfectly on our 350 before the bearing broke apart on a cylinder and we decommissioned it.
 

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I tell guys to give it a shot of ether/starting fluid to make sure it fires. The propane systems are the first thing to blame because it's different. If it doesn't start on ether, it won't on propane. Once you get the dist. in place I would bet that it will fire up. The engine draws the propane in under a vacuum, not under pressure like you would think.
 

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We have Impco parts in stock if you need them. Anytime there is a backfire it is a good idea to check all diaphragms in the mixer and secondary side of the vaporizer. Generally the idle diaphragm takes a hit first as it is the most delicate. If the timing is anywhere close to top dead center it should fire off if there is a vacuum signal to open the vaporizer.

An easy way to tell if you have a vacuum leak with a 425 mixer is to spin the engine over with the air filter off and watch the air valve assembly. If it pulses up and down while the engine is spinning then there probably is not a vacuum leak. But if it does not move there is definitely a vacuum problem or the mixer has been warped. It is possible to over torque mounting screws/bolts and warp the housing or base plate. Make sure when installing on an engine that the air valve moves freely in the housing after it is bolted on.
 

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Hi,

No start on Ford 1985 F800 370 V8 after back fires caused by a broken wire on the dist. pickup coil. My first new replacement pickup coil went bad and luckily bought 2. I rebuilt a spare Impco E series regulator and moved it away from the 425 mixer/carb as it was mounted horizontally right to the 425 (likely so air cleaner would fit) and have it now properly vertical (Impco Instructions) . The spare VFF30 seems to function okay and rebuilt my old one as a spare as it had a ripped black diaphragm. The main diaphragm in the 425 looked good (looked like a fresh silicone one) , but I replaced it anyway along with a new valve.

I missed the idle mixture diaphragm when ordering parts and thanks to this forum I checked it and it is not ripped, but stretched a lot on the end at the front of the carb. where the adjustment screw is. Could this cause my no start? The air valve doesn't move on my 425 either on cranking. I can suck on the vacuum hose to VFF30 and hear the gas going to the E series regulator and after pushing the primer hear the gas going to the 425. I tested the E series regulator the way Impco instructions said to.

I have to move the truck off the street due to coming snow and parts take a long time to come so could I maybe glue some nylon material to my idle mixture diaphragm to stiffen it ?

Thanks very much!

Randy
 
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