Hi all, i throw this question out in this thread, i know you guy’s can help me.
Since im going to get the chevy truck up and running, i remembered a small problem i’ve ignored some time now, on the truck i have a th400 auto-trans, and when driving on wood, the shifts are somewhat “delayed” and rough, i’ve rebuilt the trans maybe 3 years ago, the problem became much more noticeable.
I think this should have to do with the vacuum modulator (new), the trans otherways works as a dream when driving on gasoline.
Is it anybody noticed the same?
I know there are adjustable vacuum modulators, maybe possible to find a sweet spot inbetween?
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I would definitely see if your current modulator is already adjustable. Or get an adjustable one. When you look into the hose barb on the modulator the valve will have a flat headed screwdriver slot cut for adjusting. Rotating tighter will firm up shift points and loosening will soften shifting. I don’t think it actually changes the MPH shift points per se but it does change the firmness, how much vacuum is required to go the next gear. If you’re heavy on the throttle a lot then manifold vacuum will be high so you’d want to firm up shifting. If you’re easy on the throttle then maybe loosen it some.
Do you drive lightly on gasoline and then heavy with throttle on woodgas? Maybe this explains the rough shifting.
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Thank you Cody, that was the answer i was looking for, as for driving im gentle on the throttle on gasoline, and even more on woodgas just to avoid the hard shifting, that almost shakes the truck. Anyways there is a distinct difference between gasoline and woodgas driving.
I should definitely look if the vacuum modulator is adjustable, had not thought about looking into the vacuum connection.
It’s really good to be able to ask this stuff, auto-trans knowledgeable people are as rare as hen’s teeth, here in Sweden 
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Think about it.
Sucking a woodgas system the intake manifold vacuum will be far different.
For delaying emissions kick-in; systems back in the 1980’s used a lot of timed inline vacuum delay valves. Black on one end. Varius colored on the other end to indicate installation direction and time delay.
Impossible to buy now.
G.K. try some small aquarium inline filters to try and get the same actuation softening step. They have inline manual valves too; I’ve used for direct from the cab tuning of feed-back carburetors air blending circuits with long dead control arrays.
So . . . Maybe.
S.U.
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Thanks SteveU, as for it was the different vacuum, that messed things up, it’s what i suspected, therefor i thought about "finding a sweet spot between wg and gasoline vacuums.
But, interesting you mention these vacuum delay valves, i’ve got a box of them saved from an Opel i scrapped, haven’t thought about the function of them, only there was LOT’s of them on this Opel, just in the make between carb and fuel injection, terrible cars to work at with all the vacuum hoses, as a spider-web.
I thought about this problem right after i rebuild the transmission, i found a vacuum booster? (Direct translated from my Swedish engine books) this came from a Mopar -76? something, 3 inches dia, round, black plastic, adjusting screws in both centres. I’ve thougt about “dissecting” this to see if i can modify it to become some kind of vacuum “adapter”, reducer?
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Goran, cars with diesel engines have built-in vacuum pumps, years ago I had a Jeep Cherokee 2.1 td that has front wheels with a vacuum diaphragm, … maybe the solution to your problem would be to install a vacuum pump
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What difference does it make when you drive on gengas against petrol, on the gearbox?
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If a secure method of vacuum would be the fix, I would suggest as tone said a pump off a diesel. The Ford superdutys has a remote mount electric pump that many guys adapt for other purposes, small 12 volt, remote vacuum canister easy to place wherever needed
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Hi Jan, on gasoline the transmission works perfectly, on woodgas it “hesitates” and shifts hard/ rough.
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Goran it could be your vacuum is very high on woodgas, since air is choked to meet stoich.
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Yes, but why, what’s the difference?
Tone and Marcus, your in to something here, but i don’t think a vacuum-pump should be necessary, i think in this case the problem is there’s to “much” vacuum, (too low pressure) (hard to explain/translate?) due to the restriction the gasifier, filter, piping becomes, this affecting the transmission, wich uses the underpressure to adjust the shifting, to fit the type of driving/acceleration.
But, this truck had a diesel in it before, i remember it had a vacuum-valve, affected by the throttle to create the correct vacuum-levels for the transmission.
Maybe if i find this valve, (don’t remember what i keeped, sold the diesel engine) and fitted it to a vacuum reservoir, i could create stable vacuum levels for the transmission to use?
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Yes, it’s what i think, i’ve had a hard time to share what i mean about vacuum, low vacuum/ high, underpressure?.. even in my native language? 
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I think it would be high vacuum.
Your engine pulls the same CFM of air volume but the flow is restricted causing higher negative pressure
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I know nothing about auto trans.
Just wanted to remind you that with a closed throttle the intake vacuum is the highest = low pressure. Even pulling on the gasifer with an open throttle makes for relativly low intake vacuum = high pressure.
So, if anything, running woodgas calls for more throttle, which in turn lowers the intake vacuum (less vacuum = higher pressure) and makes the transmission believe you’re putting a lot of power into the driveshaft…and then what? I don’t know 
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Yes, high vacuum, due to suction resistance.
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A vacuum cylinder or diaphragm needs a certain pressure difference to do its job, if this difference is too small it does not perform its task, it may be the cause of the problem. There is probably data on the value of the vacuum and one could measure and compare this when driving on wood and gasoline, … 
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Aha, diff pressure! That confirms that my first instinct to stay out of this discussion (which I didn’t follow) was right.
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The higher intake-vacuum when driving on wood.
Jo, I think your explanation is correct because it is difficult to explain what is high vacuum and what is low, I am talking about the pressure difference related to air pressure 1 bar. If we agree to use absolute values, I can say that the petrol engine generates 0.3 bar of idle pressure and 1 bar at full load, but probably, as you say Jo, when driving on wood this pressure is 0.5 bar or even higher. 
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