Loss of power slant six

Engine, transmission, rear-end, driveline, fuel system etc..
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12Valve
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Loss of power slant six

Post by 12Valve »

Hi All,

This past week I have been chasing an odd issue. A few weeks ago I put a MSD BTM6 on my truck as well as a pertronix electric ignition. The issue I was having with that was that the truck would be hard to start, I later found out the timing marks on the crank were not correct. 10 degrees BTC is actually TDC, changed that then the truck ran like a champ.

This past week I noticed that the truck lacked power in gears 2 and 3 or 3 and 4 respectively. In first gear I can get the truck to rev and pull to 4,000 RPM, then in second on a pull she won't rev past 3,000. It feels like the truck surges then eventually will stall if you hold the throttle in. In fourth gear it does it as low as 2,000 Rpm.

I thought it was a carb issue but when I tore down the carb I didn't notice anything out of the ordinary, I readjusted the float height, but I adjusted it too much so it was flooding the truck, I changed the float height and now it runs exactly the same.

Today I pulled the valve cover off and barred the engine over back and fourth to see how much slack was in the chain. There was maybe a quarter inch of movement between the crank and valve train. And when I barred the engine so that cylinder 6 was at the beginning of its exhaust stroke so that cylinder 1 was on its compression stroke allowing me to measure the valve lash. Knowing that the slant six has to have these measurements done with the engine warm, when I took the measurements cold there was a two thousandth difference on both sides ( Intake .010 measured .012, Exhaust .020 measured .022) So I do not think that this issue is a cam or timing issue.

I have ordered a new Holley 2300 to convert it for a blow thru setup.

Does anyone have the slightest idea of what the issue is?

I changed the fuel filter, cleaned the screen in the tank, checked the fuel pump. Checked timing both cam and ignition both were fine. This leads me to believe that the carburetor is the issue here.

HemiJoel
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Re: Loss of power slant six

Post by HemiJoel »

I would put a tee in the fuel line right by the carburetor and run a hose to a temporary fuel pressure gauge. Drive it and see what the fuel pressure does at the time that the unwanted symptoms occur. You can run the fuel line out from under the hood and duct tape the gauge to the outside of the windshield so that you don't have to run a fuel line through the firewall.
1968 D200 4 door 318 auto power steering
1966 D300 slanty 4 speed flatbed with Schwartz dump
1968 D200 crew cab project to become a longbed cummins dually

12Valve
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Re: Loss of power slant six

Post by 12Valve »

I figured out the issue the new fuel filter had collapsed sending crap into the carb causing all sorts of issues. The carb is pretty tired the throttle bushings are shot, and now with filter elements all over inside the carb, the truck will not run below 2000 rpms. I have just bought a holley 2300 to replace it so when I do turbo the slant six, I don't have to worry about the worn throttle bushing causing a boost leak. On a positive note just incase I kill the motor with too much boost I bought a 440 with a 727 out of a motor home

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