compression/carburator questions

[skeit80]

i've had a "popping" sound from my 2.0 liter engine especially when it is idling. i had it checked out by my trusted porsche mechanic and he changed the plugs as they were gunked up and the popping sound is now gone. he said he thought the carbs may be too big for the car-resulting in too much gas getting to the engine. he closed them as much as he could. what is the largest weber carb my 2.0 liter can handle?

he checked compression and he said they were around 90 - 100 for each cylinder (using a 20 year old uncalibrated guage). i know this is low, but the engine runs well. it leaks a little oil and does burn some oil. do i need an engine rebuild?

also, i've owned the car for 3 years (only driven a couple thousand miles total as i've been restoring it) and have yet to get the valves adjusted. i don't know the last time they were adjusted by the previous owner. my porsche mechanic said there was no need to adjust the valves as the car is idling and running/driving well. is this true? how often should valves be adjusted? will adjusting the valves bring up the compression? should i be considering an engine rebuild? what's an engine rebuild cost? i'm new to this, so any help would be appreciated. thanks in advance. any good mechanics in charlotte, nc for my 914? i'm moving soon. jason

[Dave at Pelican Parts]

The max carb size is determined by the amount of airflow that the rest of the motor can support. In a 914, that is determined by the exhaust, port-work on the heads, and the cam selection. (As well as other mods that let the engine spin to the high RPMs that using lots of air will require.)

The current thought seems to be that Dell 36es or Weber 40s are a good size for a stockish 2.0 liter. Larger carbs can be used (e.g., Dell 40s or Weber 44s), but economy suffers and the low-RPM power can suffer.

If you have Weber 48s on the thing, those are just plain too big. They also work kind of like a light switch--decent at closed throttle or WOT, not very good in between (which is where most street motors live 80% of their lives). Good for race cars, not so good for street cars.

Valve adjustments are supposed to be done every 6K miles. I'd consider that an annual check (if you don't drive that much) would not be a bad idea. Remember, you're checking the valves and adjusting any that need it. And valves that are too tight don't make extra noise, and those are the ones that can burn.

A valve adjustment might bring the compression numbers up, if they're out of spec. Or it might not.

100 PSI compression is quite low. It might be wise to start saving up money now for a rebuild. A leak-down test could tell you where the compression pressure is escaping, which will give you some idea of what's going on inside the "top end" of the motor.

Rebuilds vary a lot in what exactly gets done and what parts get replaced or reused. I'd guess you could possibly get someone else to rebuild the motor for about $3500, if you looked. It seems the $5000 range is more typical for a 2.0 motor, and "extras" can easily push that price up into the stratosphere. (Like, if you want a 200+ hp motor, start with $10K and realize that you could easily spend another $10K...)

--DD

[Kevin Powers]

if you remove an air filter top and look closely down one of the throats, you should see a couple of #s, ie 28, 30, 32, etc, on the top edge of the venturi(the circular portion of the throat). i have no idea what "closed as much as he could" means. turned in the idle/air mixture needle screws? wrong approach. 44s with fact vents (36 irrc) really work poorly on a "stockish 2.0. even on a hopped up 2.0. you should buy tomlinson's weber book from cb performance (it's offered here i think or most non factory vw parts houses). you are throwing your money away paying a shop to tinker with your carbs. get the book, figure out the components and their locations, and get some numbers-main jet#s, idle jet#, venturi size, air correction,...hard to diagnose without data. lastly, what kind of fuel pump are you using?

k