Sputtering, no power under 4k rpm.

[HOLLYWOODSPIDER]

Hi, all.

I recently replaced my cylinder head with a fresh one (valve job, springs, seals). I had to when a machine shop turned my head into scrap metal. The shop is Modern Engine in Glendale. They suck, do crappy work, and only guarantee machining if you pay them to install. I recommend going anywhere else for machine work.

Ever since I got it back from the shop, it's been slow in the lower rpms and pops like a popcorn popper. Out the carb when I accelerate, out the tailpipe when I let off the gas. It's been like this for a week.

Starting today, I have no power at all under 4k rpm. I'm talking 0-30mph in 25 seconds. I have to rev it over 4k before I let the clutch out or I go almost nowhere. I took out the idle jet (on the side by the driver's fender), blew it out, and reinstalled it. Now it won't idle, either. Before I did that, it was at least idling smoothly.

It sucks to spend a lot of borrowed money on your car and then have it run like crap.

At first, I thought the mechanic accidentally installed a racing cam (still haven't ruled that out) because it runs great at fully open throttle. Now I'm leaning toward a carb issue. Anybody want to chime in??

Timing is set at 10 btc. It's a 78 1800 with a 1981 electronic ignition, vac advance unit is new but not attached to vacuum.
Carb is a 32/36 with electric choke.

[So Cal Mark]

sounds like the cam timing is off a tooth or two.

[racydave]

Ive done it myself, like Mark says...

[Jazgeek]

I had a similar problem myself but it turned out to be a restricted fuel line.

fuel injected here so it may not apply to you.

[mdrburchette]

Could be a vacuum leak? My 71 would lose power, pop from the carb and backfire from the exhaust. I found the cap I had put on the intake to close off the vacuum had blown off.

[HOLLYWOODSPIDER]

Thanks for the suggestions on what to check. I've been out of town for the holiday, and today it's back to work. Hopefully I can find some time this week to start diagnosing. I'll let you all know what I find.

[HOLLYWOODSPIDER]

Always wanting to find an easy fix, I checked the vac connections first. Bingo! The vac line to the diverter valve was cut in half. Huge vac leak. Problem solved? No. Same symptoms as before.

More info:

Gas gauge hasn't been reliable for years, so I use my trip odometer. I've averaged 20 mpg in the past. After running out of gas twice, I have figured out that I'm now getting 10 MPG!!!!!!!!! 10!!!!!!! That's less than I get in my 66 Thunderbird that weighs 4700lbs and has a 390 V8.

[mdrburchette]

You're getting 10 mpg after the head rework or was this going on before? What do your plugs look like? Are they fuel soaked? Do you have gas in the oil? That gas has got to be going somewhere. Even with my performance upgrades, I can spend an hour tuning my dual idfs and still get 20 mpg out of a tank.

[HOLLYWOODSPIDER]

Before the head swap, I was getting about 20-23mpg. I could get to 200 miles on the trip odometer from a full tank. I've put a total of 12 gallons in the tank since it came back from the shop, and I ran out again this morning. I've got 121 miles on the odometer since the head swap.

I have not checked for gas in the oil... how would it get into the crankcase?

Mechanical fuel pump is 2 years old, and the fuel filter is mounted just before the fuel pump. It's a clear glass filter, and I can see that it's clean and the fuel is flowing... when it's not out of gas.

[mdrburchette]

It can either come from a faulty fuel pump or from the carburetor. If you have gas in the crankcase you may need to check the float on the carb to make sure it's not full of gas and no longer bouyant or it's misadjusted. Your problem may be fuel related.