Any Carburetor Tuning Tips for a first timer?

[Scott Danforth]

Stock gm 350 with 76cc heads about 140-150 psi. With vortec heads, about 150-165psi
130 is a marginal motor. 110 is a pull it and rebuild it motor

 

[alldodge]

You should be able to get a tester from the local auto parts store as a loaner.

Compression should be close to 150 psi. Check compression with the coil disconnected, throttle at WOT, and crank at least 4 turns

 

[stancalame]

Scott
Just re-read a post. When I turned idle screws all the way in, the engine stumbled and one one occasion died when I didn't back off in time. Can that confirm power valve is not blown? It seemed the screw on starboard side had little impact on rpm and vacuum.

De S any of that suggest anything?

 

[alldodge]

It seemed the screw on starboard side had little impact on rpm and vacuum.

Tells me your carb idle circuit is clogged on the side, or your seat and needle are damaged. Remove the needle and see if its damaged

 

[stancalame]

Thanks AllDodge I'll do it. I had late night working on boat. I decided to tear into the control box since I finally got the shifter on the spline. I put it back together wrong the first time and should not move. However I would not be defeated so I disassemble reassemble and discovered my error. I'll be reinstalling connecting cables next trip. I am encouraged because after greasy oh Rado and shift cable they move very easily by hand. I'm hopeful my issue was bad but just in fact I saw a note in the manual regarding throttle position.

 

[stancalame]

I'll apologize up front for video. Not sure you can hear me over engine noise but thought I'd share incase it makes a difference. I've read a bunch of info on Power valves and I'm not surpised I get differing opinions on likely hood of damaged valve. There is a camp that says its alm its guaranteed after back fire and a camp that says its rare. I'll have to pull the carb to clean it anyway so can check it then. Found internet post " Carb Tuning the Scientific way" by Gerard Forgnone. Look pretty comprehensive, 31 pages with pics...!

One video is connecting the tach - It was weird because instructions said one lead goes to BAT NEG and one to the postive terminal on coil however I could only get a reading on the Tach by connecting to Negative side of coil. Not sure what that implication that has on any of the information I took from the TACH. Maybe its worthless. What do you think?https://youtu.be/00KEFptsH5s

One video is RPM comparisions between test tach and boat tach and a really poor attempt to show the timing mark. Doubt you can make anything out.https://youtu.be/JJyUaKAwDpU

One video is vacuum gauge and tach. https://youtu.be/b5BiXTLzMDs

 

[alldodge]

I think your boat tach is more accurate then the hand held. Was you boat in the water or on muffs? If it was on muffs then need to recheck the in gear rpms, it needs to be under load.

The vacuum may be low from where your getting it, knowing the compression will clear it up

 

[stancalame]

Boat was on a lift in the water. I realized the morning that my trim was up and I know thats a "no no". I assume that would not make a difference in vacuum. I pulled vacuum right off the intake so it should be accurate. Brand new vac gauge. I'll post back with compression numbers may be after the 4th. Hope it will before, but I'm running out of time and have family looking forward to being on water.

I'm curious if the adjustments I made on timing and carb will make a difference, better or worse once I take it out and run it.

 

[alldodge]

I'm curious if the adjustments I made on timing and carb will make a difference, better or worse once I take it out and run it.

They always run differently in the water as compared to on a hose. It should be fine IMO

 

[stancalame]

I did not get to get on the water this trip, spent whole time working on boat to reinstall and adjust control box and shifter then engine started running hot however I did pull plugs and check compression. PHotobucket not cooperating this morning so I cant post plug pics. All plugs had signs of runnig rich and only a few had the brown coloration that is desired. So this confirms carb too rich. I cleaned with carb cleaner but could not really remove the black coloration. There was no significant carbon deposits. When I'm able to understand and solve for the overheating I can get back to tuning ( Iposted new topic last night). I'm curious to see if my last adjustments made a difference. If I take it out and run for an hour or so will that be enought time to read the plugs again? Although I cleaned all plugs with carb cleaner the back color remained. I know I'm looking for the brown color that shows good combustion

All cylinders tested at 150 PSI so I guess my 15-16 in Hg vacuum is normal for my engine.

 

[kenny nunez]

You can get a kit to protect the power valve from Jegs or Summit. The kit is cheap and easy to install. Basically it is a check ball.

 

[stancalame]

Here are pics of my plugs. based on images of what good looks like on internet, I'd say I'm im running way rich. I ran into over heat issue so got distracted from this. I solved over heat and ran the boat several hours. It seems to run good but I have not checked the plugsd again. Next trip out I'll take power valve just in case. I need to run the test by turning the mixture screws all the way in to see if enginer stubles. I ran out of time on 4th of july and had folks iching to go out. I want to solve for the black plugs!

 

[alldodge]

Your plugs look more carbon fouled, so a bit to much fuel, weak ignition system/timing, gap and/or incorrect heat range on the plugs

 

[stancalame]

THanks for the tips. I'll start with power vavle on carb. I'm confident in the ingition timing because I've checked it multiple times with light and on I'm right on 8 to 9 BTDC which is specified, plug gaps are correct, double checked them this weekend.

Not sure how to test for weak ignition system or heat range on plugs.Any clues there?

 

[alldodge]

Weak ignition would check spark with a gap tester
The heat range is you should have the correct spark plug type for the motor

 

[stancalame]

Got it. I'll google gap tester. Please feel free to share any good links for my edification if you have them. Especially if if there is a way to make your own or get them inexpensive! Two kids in college, dont you know

 

[alldodge]

Like this, can be found at most auto stores
https://www.amazon.com/OEMTOOLS-250...coding=UTF8&psc=1&refRID=5RN36S3JC8RZAN7DBW1A