Engine loses power under load. WTF??

[Rothu]

Ok, I have a 1976 Bedford fitted with a holden 186.
I replaced the head with a 173, which made it run bloody well! Then blew a head gasket.
Got the head totally rebuilt, and put on extractors. running like shit ever since.
It idles fine and smooth but loses all power under load. I'ts running on LPG, changed the LPG convertor, rebuild the carby, changed HEI leads, changed the coil (has constant 12volts). Still loses all power under load. What the hell could it be??

[Marishka]

whups , accidently modified ur post , but fixed it and posted reply as shoulda done first,

are ur valves adjusted correctly?
looses power under load? up hills?
can u get full power on a straight or does she get to 3/4 throttle then there,s no more?

sounds like she,s reaching full gas flow and might need more? = is it a 6cyl converter. 4cyl ones might not give enough flow.
 
or valves may need adjusting?= might not be gettin full compression.
= have u done a compression test? valves might not be sealing correctly.

just 2 thoughts at the moment.

[Worzel]

does it only loose power on LPG or don't you have dual fuel?

[Rothu]

It is a new 6 cylinder gas convertor. Can't get it going on petrol just yet. Been on gas for years.
the head was totally rebuilt. Bled the lifters.

Idles beautifully! But as soon as you apply load it dies badly. Barely makes it up hills. I've tried 3 other coils, same situation.

What gets me is that it was running amazingly before the head gasket blew. Since then, I've tried new head gaskets, got the head checked again, all valves perfect. Compression on cylinders are 110,120,110,80,150,95. Slight gas oil on number 6 plug, went up a grade.

[Marishka]

can u hook petrol up to her?
 if u have the carby but no fuel tank just use a boat fuel tank to get her runnin on fuel to see if she still has the problem under petrol.
 that way u could rule out engine/ elec/ignition problems and then know its gas related.

it will take a boat fuel tank, elec fuel pump(if mech one is removed), and a length of fuel line.

make sure all fuel lines are clamped tightly and dont smoke in the cabbin when ur doin the test.

ive done this test a few times on gas only vehicles still fitted with a standard petrol carby.

[ben]

wow you recon 173 head on a 186 goes well.. then im gunna put a 173 head on a 202. should go real hard:)

another thing to try is make sure engine timing is right. and not a tooth out.

we had a motor running with the dizzy a tooth out, but had no power, we could rev it and all that it was just like it was strangled.

cheers
ben

[Merlin]

sounds like the bottom end is worn out it should have around 160 -180 psi compression with a 173 head on a 186 and you shouldnt have more than 20 psi fluctuation between cylinders  try sticking oil in the bores and crank it with no plugs for 4 or 5 revs then do a compression check if it improves then ya rings are shot    Cheers Neil

[Marishka]

if its not mech/elec,
 sounds like its gettin enough air but not enough gas.
might be a silly question but u checked that a hornet hasnt built a mud nest in the gas line from the tank to converter or anywhere else while the converter was out,

believe it or not its bloody happened to me.
 and had the little b#@$ards build nests in whipper snipper exhursts 2, took me about a hour to find it, lol. gets the thinker mass thinkin.

ive had them build nests in = fuel lines, gas lines, carby,s whipper snipper exhursts, spray guns, air tools = they just dont care as long as the holes the right size.

disconnect the converter tank feed line and get someone to turn the tank on n off quickly to test the feed line. make sure theres no one smokin and disconnect the battery, etc, no ignition things near u like things that spark.

if there,s a hornet nest it should blow most of it out, but bear in mind that just 1 tiny fragment on forign material can cause a blockage.

especially in the final gas injector, its a small hole.

[ben]

hey merlin where do you find out what sort of psi you should expect from engines? or do you just know:)

ben

[Marishka]

google it ben, the psi compression of a motor, its amaising what google dosent know.

p.s. its not so much the compression but the fluctuation between cylinders.
but i think that overall compression should get no lower than 90 to 100psi
ive got a 2 stroke 15hp evinrude outboard that has 80psi in both cylinders and still runs smooth as. and it lived up the hawksburry river at brooklyn before i bought it being flogged by a young guy wake boarding every possible chance.

its on my 11ft savage tinny and still fly,s.