Dual Fuel Conversio Air Flow on Gasoline

Propane, Butane, LPG, GPL, C3H8, C4H10
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crustysarge
Posts: 4
Joined: Sat Jan 30, 2016 12:47 pm

Dual Fuel Conversio Air Flow on Gasoline

Post by crustysarge »

Hello,

This is my first post. I am stationed in NE Italy and have a 1984 GMC G20 camper van I am restoring. It has been around in Europe and I bought it in Holland. Specs are GM stock Goodwrench replacement 350 and a TH400. I recently did a Vortec head conversion and put a mild Lunati towing cam, Weiand intake, Summit 600CFM carb (vac secondaries), headers and duals.

Van had a GPL conversion done in Holland and had Impco 300 10/20 on a Quadrajet and Model E installed with three under body tanks with 160L capacity, they custom built a small gas tank to keep dual fuel but it was seeping. Italian DMV required me to redo the system as it was pretty hillbilly, I was kind of shocked at some of the poor install work done in Holland, the EU states are usually pretty anal about this stuff. Anyway, I installed a new 35 gal gas tank and two Stako 52L propane tanks under the body with dual filling and dual feed valves IAW local law and rebuilt the 300 and Model E. Before trying anything I just ran on gasoline (local regular is 90 octane) and the van runs great, really strong. I installed the 300 and did not run propane, just gas, starving for air. I triple checked the Bowden cable and cam for air and all is good. I put a bigger filter I made out of a 14X3 and no change. I then tried a 50/70 300 but no difference. After inquiring around I decided to try a new 425 with a 90 degree adaptor. Worse, has no power at all, I double checked the vacuum pull and the 425 is wide open. There is just not enough air flow. I think the aperture on the 425 is smaller than the 300 to be honest. I live at the foot of the Dolomites and the van could not even make it up the small hills last night with the 425, I had to weave around the village on side roads to avoid the pull uphill.

So, after all this work I am ready to throw in the towel and sell all the parts and run gas only. I wanted propane as it costs 60% less than gas and is readily available in Europe but I need the gas for backup and towing power. I need more air flow on gasoline than the mixer is giving. The only thing I have not looked at is if the carb secondaries are not opening but I will do that today or tomorrow with the wife driving and the engine cover off in the van so I can see them.

Any suggestions or ideas? I was contemplating some kind of mod on the adaptor that I could open and close to allow more air in but looks painful. Can the 300 be modded for more air flow on gas?

BigBlockMopar
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Location: Netherlands
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Re: Dual Fuel Conversio Air Flow on Gasoline

Post by BigBlockMopar »

Legislation in Holland is soft when it comes to propane conversions.
Basicly everything has to be 'OK' and dated properly at time of installation and first 'DMV'-check.
But after that you can change just about anything on the propane system because parts dates won't be checked anymore at the yearly or two-yearly 'APK'-intervals. Only obvious signs of damage to filling hoses and tanks for instance will be checked.

Why would your wife need to be driving while you check for secondaries operation? This can be checked manually with the engine off unless you have a vacuum secondary carb.

I'd like you to try and drive on LPG-only with the 425 and it's own throttle plate for a while, so no double-up system with the gasoline carb along with it.
Adjust timing and ignition advance curve accordingly and report back if you still need more power for the hills.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

crustysarge
Posts: 4
Joined: Sat Jan 30, 2016 12:47 pm

Re: Dual Fuel Conversio Air Flow on Gasoline

Post by crustysarge »

They are vacuum secondaries so best way is watch on the road, easy to do in a van. I don't have a spare throttle body for a square bore intake to run solo on LPG so I need to scrounge one up.

I need to understand why the airflow is so restricted on gasoline and what, if anything, can be done to help.

Italians are extremely severe on the GPL thing in this region and double check everything, they even wanted a certificate from Impco the 300 was "CE" approved, luckily it is.

John

C3H8
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Location: Winnipeg, Canada

Re: Dual Fuel Conversio Air Flow on Gasoline

Post by C3H8 »

My understanding is that you first problem is that once you install the propane mixers the engine does not run properly on gasoline. Is this correct?

If so the first item to look at is the carburetor bowl venting and then the choke system. It appears the mixer may be causing a vacuum on top of the carburetor affecting proper venting of the gasoline bowls slowing the fuel delivery to the engine. This can be caused by the style of adaptor or some discrepancy in the mixer ring that lifts the air valve out of the way. Once lifted there is no reason for it to interfere with airflow. Depending on the choke on the gasoline carb you also need to look at it. Most chokes are not affected but I am not familiar with summit carbs. Some four barrels in NA had choke linkages very close to the outside edge of the carb and the adaptor could cause these to jam when it was installed keeping the choke from opening.

Since you changed to a 425 I suspect some interference with the adaptor. All the mixers you tried should work with minor differences in power. Typically the 300A50/70 was the best choice for ease of installation.

There have been thousands of these older GM vans converted to propane without any major issues, however most were with the original Rochester quadrajet. They were done in both single and dual fuel. As mentioned performance was better on the straight LPG units as the timing could be adjusted to suit propane. That being said there is no reason your engine should not operate on gasoline and propane.

I looked at the Summit page and I see it is very similar to a Holley Square bore. One other item to check is the vacuum pressure to open the secondaries. We used to have issues with the Ford 460 motorhomes in Canada once the mixers were installed. After hours and hours of testing we discovered the secondaries were very slow to open once the mixer was installed. Changing the secondary spring pressure solved the issue. I'm not familiar with all of the characteristics of Summit carbs so I'm not sure how sensitive they are to changes in vacuum. From your description I would be inclined to take a close look at secondary operation on LPG and bowl vent on gasoline.

Don't give up. They do run good once you find the true issue. It is possible to vent the bowls by routing a hose from the vent through the adaptor if necessary. That hose would have to be routed to the air cleaner plate just above the mixer to ensure any gasoline fumes are drawn back into the engine.

kuchen
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Joined: Tue Oct 25, 2011 12:38 pm
Location: Stuttgart/Germany

Re: Dual Fuel Conversio Air Flow on Gasoline

Post by kuchen »

I confirm that changing the spring tension on the secondary air flaps to make them open earlier does the trick on a Q-Jet with an Impco 300a.
They are basically pushed open by the differential pressure, and that is lowered by the pressure drop caused by the 300a.
I run an Olds 307 with the same setup without any noticeable power differences. It is equipped with the bigger one of the two available open filter elements, though (Pics see your car section of this forum) and is tuned with a WB O2 sensor to run lambda 0.95 on LPG at WOT (0.85 on gas with the stock Q-jet)

crustysarge
Posts: 4
Joined: Sat Jan 30, 2016 12:47 pm

Re: Dual Fuel Conversio Air Flow on Gasoline

Post by crustysarge »

Hey guys, been bad weather and I have not had time to work on it. I will try later this week and see what is happening with the secondaries. Have to go to Tuscany this week for work at Camp Darby so when I get back I I'll get on it.

crustysarge
Posts: 4
Joined: Sat Jan 30, 2016 12:47 pm

Re: Dual Fuel Conversio Air Flow on Gasoline

Post by crustysarge »

Thought I would follow up on this. I discovered the 425 mixer was momentarily closing during acceleration on gasoline due to the engine going to zero vacuum. The 425 uses engine vacuum to open/hold the port for air flow. So after thinking about I dug out an vacuum canister I had laying around and installed it and it really helped, in fact it cured it IMO on the gasoline side of things. Then I went back and moved some things around in the engine compartment (G20 Van is tight on space) and managed to install a 6" long K&N filter to replace the 3" Mr Gasket and wow, LPG and gasoline run great now.

I logged about 5,000Km this summer on propane and 2,000 of that was towing my race car up and back across the alps to Germany. Never missed a beat on propane and averaged about 9-10mpg which is not bad considering the van weighs 6,500 and the trailer car is another 5,000 or so, speeds at about 50-55mph.

C3H8
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Location: Winnipeg, Canada

Re: Dual Fuel Conversio Air Flow on Gasoline

Post by C3H8 »

Great that you found the solution. Also great that you reported back. This is something I should have thought of. It's been so long since I have physically been under an 80's hood I forgot all about the vacuum canisters commonly used in that era. I'm sure the info will help some of the other members. We always appreciate receiving the solution to a problem even when it is something we did not think of.

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