Carb throat popping at speed, New Problem!!! Options

[LetoAtreides]

Ok, so I know I've posted this before, but I still cant figure it out, and I need to know all of the possible circumstances under which this condition occurs.

One of the throads on my dual carbs is excessively lout at speed. It sounds like pop-pop-pop.

Any ideas?

[r_towle]

well, its lean.
How it has become lean could be alot of things.

Start with the valves and make sure they are adjusted correctly.
Car will never tune without them set properly.
Also, make sure the manifolds are tight and leak free.
Make sure the carbs are properly synced and the exact same air/fuel mixture is going into each venturi...
How?
Remove the throttle linkage to start with..
Check each carb with a syncromter with the linkage removed.
This is when you use the air bleed screw on the bottom outside of webers to match each port to the same flow.
Do all four venturis with no linkage...they should draw exactly the same.

Then you re-install the linkage and sync two carbs to each other.

Other things to consider.
At speed you may be cruising but using the idle circuit only...or a blend of both the idle circuit and the main.
So, clean all the jets on that venturi..see if one or more are clogged and starving it for fuel.
check the float level on both carbs...make sure they match.

Finally, read the plugs...you need to learn how to read plugs properly to ensure you dont burn a valve.

Rich

[drive-ability]

Ok, so I know I've posted this before, but I still cant figure it out, and I need to know all of the possible circumstances under which this condition occurs.

One of the throads on my dual carbs is excessively lout at speed. It sounds like pop-pop-pop.

Any ideas?

(IMG:style_emoticons/default/wacko.gif) Hey I'm a V-8 guy and If my engine did that I would be wondering about a couple of things. one my cam shaft, having a flat cam will cause "exhausting" out the intake manifold. In your case with a two carb 4 cyl such an action would raise havoc with performance.
I also would wonder about your valve adjustment but I'm not sure this would fit.
Sure you could have a defective dist cap etc...

[Elliot Cannon]

If it accelerates OK but starts popping when you are at cruising speed, it could be clogged idle jets. It doesn't take much to clog them. The orifice is so small I use a jewelers loupe to check them. If they are clogged, you can just blow them out with air and reinstall. If you take out the idle jets, DON'T drop or otherwise lose the little "O" ring. AAAARGHH!!

[ArtechnikA]

also - check for bad, bent, or misaligned linkage.

check for synchronization at idle with a SynchroMeter and then eyeball part- and full-throttle.

If the carbs open at different rates because of linkage lever arm length or angularity differences, they'll go out of sync. That can cause this.

And this presumes that your valve adjustment and ignition timing are spot-on.

Many if not most 'carburetion' problems are actually elsewhere.

[LetoAtreides]

Update!

I adjusted the linkage on one side, and immediately, the sound switched sides. So I guess it was just the linkages not pulling the same way. So I've adjusted the linkages and now the sound appears on both sides. My next step is to adjust the mixture. I checked the plugs and they were all black, so I guess I'm running too rich.

Just to be clear, the sound appears both on acceleration, and while cruising. Although I haven't driven it for a while, out of fear that something will get screwed up. However, last time i drove it, it was at full power as far as I could tell.

Things I have done so far (in no particular order):
-Throughly cleaned and air-blasted the component parts of each carb.
-replaced and adjusted all ignition components.
-adjusted valves. (many times)
-timed.
-replaced mixture screws, springs, washers and o-rings. (one screw was bent)
-adjusted float on one of the carbs. (it was overflowing)
-checked for vacuum leaks.
-Synched the idle.

The carbs are DellOrto DRLA 36's

[LetoAtreides]

So i think I've pretty much fixed my popping problem, although there may be a new problem. When I am going up a hill and only at pretyy high revs, there's a significant clattering in my engine. I don't know if it's mechanical clattering or some sort of backfire, but it doesn't sound good. Also I can smell burning oil when this happens, yet my compression is still good. Is is spilling out onto some part of my engine and burning? What's going on?

[r_towle]

What is your timing set at???

Rich

[LetoAtreides]

I timed it using an electronic tach. I rotated the dizzy until I got the highest rpm.

[ArtechnikA]

Normally I am a very patient person but this has put me over the edge a bit. Someone else better at working through the basics, especially on a T-IV, wil have to take over here.

Leo -

At least part of your earlier popping and your current problem is the mixture trying to explode back through the intake because once you get a real timing light on that puppy you will (I hope) be horrified at what you discover.

Please don't drive it until you get this fixed.

What you are experiencing is called DETONATION for a reason. It is the sound of engine destruction in progress.

You can static-time it "close enough" to be safe with nothing more than a 12V light bulb on a couple feet of wire. A real timing light would be better.

There are A LOT of threads here about how to time an engine.

Max speed at idle isn't in any of them. I don't know where you came by that technique, but it's bogus (at best).

[Mark Henry]

I understand how Rich feels, the T4 doesn't like being fuched with, any lean/too much advance issue can turn it into a paperwieght real fast.

Linkage is the number one problem with dual carbs and it's always user error.
Timing 7.5 static or 28 full with a strobe, guessing is expensive and dumb, I know a heck of a lot more than most and I don't guess.

[LetoAtreides]

Sorry for testing your patience, ArtechnikA, let me explain myself.

I'm 19 years old and learning to work on my own car. I learned this supposed timing technique from my neighbor who has worked on old cars his entire life. I don't have the cash to take it into my mechanic, so I have to learn to do this myself.

I static timed the car (and subsequently shocked myself with my home-made timing light). I timed it to the advanced mark on my fan. There's no change in the carburetor noise. Tomorrow i will borrow my neighbor's timing light and time it.

Is there any way to tell if my engine is permanently damaged?

[ArtechnikA]

I'm 19 years old and learning to work on my own car.

BTDT, believe me. I'm sure many if not most here have been.
However, when I was 19 and just starting out with cars (VW's first, fairly quickly into old Porsches) there were no resources like the Internet and this site. We were happy to pick up a copy of John Muir's 'Idiot Book' (I started -before- that classic reference...) and maybe a Clymer's.

The are probably a few dozen threads on this site alone about 'How do I time a 914.' So there is really no excuse for not doing one's research first.

So - moving on...

I learned this supposed timing technique from my neighbor who has worked on old cars his entire life.

Specifically, old WATER-COOLED cars with a big carb in the middle. You could maybe get away with it there if you were careful.

I static timed the car (and subsequently shocked myself with my home-made timing light). I timed it to the advanced mark on my fan.

Alrighty - plus or minus not enough to matter, you're running about 28 degrees initial and 36 total. WAY TOO MUCH!. Go static-time it a smidge before the TDC mark for #1.

Is there any way to tell if my engine is permanently damaged?

Bring -everything- to book specs and listen real close. A compression and/or leakdown test would confirm findings if you had obvious bad running or clouds of smoke. We'll hope for the best.

Seriously - DO THE SEARCH here on timing your car. There are threads in great detail about the process.

Mostly, I don't do T-IV's so I am really unqualified to provide additional details, besides which, they're already out there. I got into this thread because of the induction linkage aspect, which I -am- qualifed to explore.

Good luck.

[LetoAtreides]

Thanks for the info.

Go static-time it a smidge before the TDC mark for #1.

How much is a smidge? is it a matter of a few cm, or a few mm?

And after that, Do I time it to that same spot with the timing light?

[r_towle]

I will try to clarify.

Step one.
Static time the motor.
this gets your motor times close enough to correct that it will start, run and not harm the motor (like to much advance will)

Next Step
Take your timing light and set the timing via the manual to the correct settings for your specific motor...I assume you have at least a haynes manual, yes???

Pelican parts has some decent technical articles to read.
This forum and the 914Club forum have loads and loads of info on how to diagnose and set your car up to run perfectly.

I will go over a few steps to do IN ORDER to obtain a perfect running car.

Set Valve lash to spec.
Set point gap to spec
Check and verifiy points with a dwell meter and get the dwell perfect.
Then set timing on the car.

If you do any of the steps after you set timing.,...you need to reset timing.

Rich

[LetoAtreides]

Ok, cool.

I checked the points and dwell angle very recently and they are to spec.
I've also very recently adjusted the valves.
So all i have to do is the timing, then.

I guess my question has to do with ArtechnikA's post.

you're running about 28 degrees initial and 36 total. WAY TOO MUCH!

The Haynes manual says that I should set it to 27 degrees advanced for the 1.7 engine. So is it really too much, or is it different with carbs? So which is right?

I also should state that I have no vacuum advance.

[ArtechnikA]

27, 28 - that's probably as close as most of us can get - within a degree.

ALL the specs are for TOTAL ADVANCE.

You set you INITIAL ADVANCE (what static timing sets...) to the TOTAL number.

The INITIAL (static) gets you close enough that the engine will start and warm up safely -- prerequisites for setting TOTAL advance with the engine running.

7° BTDC ought to be 6-8mm before TDC. It's just a guideline.

In the many threads on timing a T-IV it's noted that you should IDENTIFY and CLARIFY (white paint or something) the total advance mark on the fan. Sounds like good advice. (6's are timed at the flywheel...)

27-28 degrees TOTAL advance - this will be at some elevated rpm, like (I AM GUESSING - I DONNO T-IV's) 4500.

Don't worry too much if you don't understand the concept of distributor advance yet - lots of people don't. But if you're going to be tinkering with it, you need to know (and follow) the right procedure.

[rhodyguy]

i suggest you limp or tow your car to High Perfomance House in redwood city and have rich bontempi setup your carbs and time the ignition. then leave things alone and enjoy your car. or ask Mcmark to do a mobile service and pay the going rate (you can watch, stay out of the way, and learn). often it's to your advantage to loose the small battle but win the war. burn up your engine and it's total defeat.

k

[LetoAtreides]

i suggest you limp or tow your car to High Perfomance House in redwood city and have rich bontempi setup your carbs and time the ignition.

Yeah, I've done business with Rich, and he's never failed me yet. However, as I stated before, I've run out of cash working on this car (partly because I bought a dud engine in the past). Even if I did, limping is out of the question since there's a mountain between me and HPH. I'll probably have him do the fine tuning, but until I get it drivable, I've got to do it myself.

[LetoAtreides]

Ok, so I timed my car by the book, and it idles a lot smoother now. However, I'm getting more backfires out of the carb throats upon revving than I was before. So. I adjusted the valves (again) and re-timed it to no avail. There were no stuck valves or pushrods. I also ran a compression check and got 140 across the board.

There is a strange whistle when I shut of the car, right when the engine stops running, though.

Any ideas?