Based on my own experience from years ago, a flat cam will cause backfiring. This was in a SBC V8, but a poppet valve engine is an engine, right?

Thanks for the reply Walt. It's something I've considered, but in theory, since both sides of the engine share the same lobe, I would have the issue on both sides, and a few people have already weighed in and doesn't think this is the problem.

Here is my logic for the weak valve spring:

Compression test is fine, and leakdown is fine. But on a compression test, we're only rotating the engine very slowly in relative terms, so the springs still have plenty of time to push the valve back closed, and as long as the valves are seating fully, there is no reason that the leakdown shouldn't be fine.

So what I think is happening is that one or both of the springs on #3 isn't supplying adequate force above several thousand RPMs, the symptoms of which simply don't show up on a compression or leakdown test.

I know this probably frustrates the hell of you guys, me asking a ton of questions, but as a student of science, I feel the need to find out the cause of a problem before I move on to other options. I'm searching through this to learn more about troubleshooting/diagnosing engine issues so that one day I'll be able to tackle a rebuild on my own.

Like I said, I have a spare engine, but I'd like to work through this problem to the end before I put it in. Then again, it's probably better to swap engines, and then I could pull the heads off of this one to better diagnose what's happening. What would you do?

A broken or weak valve spring could very well be the culprit. I had a BBC with big hyd roller cam that would chew up outer springs. It would present itself as mild miss or popping (not harsh like a backfire). I chased my tail on that one through the ignition system first. My headers kept burning up my spark plug boots which compounded things. I sorted that with ceramic boots. Try hooking up a vacuum gauge to manifold port and see if the needle is smooth or erratic. erratic would suggest valve spring issue.

He's carbed the needle will bounce on the 2 throat carbs. No plenum to smooth out the. Intake pulses.

You could have had the other motor in and running by now........

My bet now is bent valve or broken spring

Hello George ,
If your up to it I have a few things that might help you out . I have a Full set of
springs ( low milage off some 1.7 NOS heads I picked up/I dodn't know why I saved
them)

Springs ,Retainers,keepers,& the valve stem seals (still flexiable ) & the tools
to them while the engines in the car , just pay for shipping & return the tools
when you finish, please ( no rush ) I hope this might help you Bill D. let me know I'll send it out tommorrow ,

Interesting... I've never used valve stem seals.

The generosity in this place never ceases to amaze me. Bill has already sent me a box of spare goodies before, and now this.



PM sent... I report back on whether this fixes it or not on an otherwise healthy engine.



I hope you realize what a great thing we have going here. Truly an amazing group of people.

Bill's tool and parts came in today and I can't wait to try to swap out springs this weekend. Thank you again Bill! While I'm at it I'm going to try to decipher whether the valve(s) are actually sticking in the guide too. I heard this occasionally does happen, especially with an older engine. This is why I'm curious about the possibility of a sticking valve:


https://www.youtube.com/watch?v=XExnP7wpafo




This was when the car was completely cold, and it happens almost everytime I try to start it when it's been sitting more than a couple of minutes. Does it tell any of you anything else? Again, it runs fine otherwise, until the backfiring issue.

If nothing else, the video is kind of funny...

What was the outside temp for that start ? I can't imagine it's too cold outside wherever you are.
I can't see your throttle linkage to see if you are pumping the gas or not.
Just like 356's or any other carbed car with no chokes you would probably get good results by pumping the gas once or twice, then with you foot off the pedal crank the engine.
When it started it sounds pretty good.
This may sound contradictory to my last comment but have you tried changing the #3 spark plug to another cylinder ?
PS please don't kill me if I'm way off, I haven't read the entire thread

If you can pull a plug off and it isn't affecting performance then I'd be really looking at firing order. That start up seems like leads going to wrong cylinders
Have you tried stopping by a mechanic and getting one to take a spin around the block? Surely worth a couple of dollars if they can shed any light at all as to what's happening
My money is on firing order or timing
Edwin

I think I have to agree that the firing order has to be off in some manner. To kick back through the carbs like that just isn't right.

Below is a picture of the firing order on the distributor cap, as I have it set up. I'm pretty confident this is correct, but you can take a look for yourself.

Time for a 6.

Try this remove the cap and turn the rotor my hand and see if it returns. This will check the mechanical advance. It may be stuck.

As mentioned before in this, now quite lengthy thread, the mechanical advance is operating as it should (recently cleaned and relubed). But thanks.

Have you check the resistance of each plug wire? They should be 5k ohms.

Move every plug wire on spot clockwise.

And reset the timing by turning the distributor clockwise till the edge of the vacuum can , the small top portion, lines up with the wire clip on the oil filler..
That should be close enough to start it and get a timing light on it.

Have you got the springs changed out yet?

Pulled both springs on the suspected cylinder (#3) with the handy tool that Bill sent me. Pulled the exhaust lifter as well to inspect - couldn't get the intake lifter to come out due to the ridge that forms.

Long story short, I compared the springs with known good springs and they look fine - same height. The lifter showed a little sign of pitting, but nothing major. I'll post pictures of that once I d/l them off my GoPro.


Took a drive, and it didn't make a difference.

Then went to Rich and Tim's suggestion about rotating the plug wires one spot clockwise. Tried to align the dizzy the way Rich suggested, but now it won't fire at all. If I continue moving the can a little further, I can get an occasional pop/loud bang out of the exhaust, but it won't fire. Here's what it looks like currently:

Nevermind. I got it by going back to the basics and setting it at TDC #1 and turning the dizzy till the rotor pointed at #1 wire. Which now means the vac advance can is wedged almost up against the tin/oil pressure sender wire.

Drove it around the block, same thing.

Are you sure you guys didn't mean rotate the plug wires counter clockwise?

Dude, time to pull the heads of and look for cracks.

Or swap out the motor.

remind me, what distributor is that again?

How are you determining TDC at number one cylinder?

Can you post a few pics of this process?

Every stock distributor is set as I stated...or damn close to the pic you posted above with the number one plug wire nearest the fan.

It still seems like an ignition issue to me....considering everything you have done so far.

Rich

Vw505, I'm getting dangerously close to that. I've got a spare engine, I just want to know WHAT the problem is before I pull the trigger, since compression and leakdown numbers are both pretty good. However, dicking around with this thing has mostly just wasted my time and money so far.

Rich, TDC determined via the markings on the fan and verifying that both intake and exhaust valves are closed on #1 (rockers rocking).

It's an SVDA distributor - more info here: (scroll down about 1/8 of the way, or search for "SVDA.")

http://www.ratwell.com/technical/FAQ.html



Could you elaborate as to why moving the spark plug wires 90 degrees would potentially solve a backfire on *one* cylinder though? In theory, if the plug wires are moved and the dizzy moved along with it, nothing else changes on that one cylinder, correct?

I would suggest that at the point of TDC you have the valve covers off and see what valve opens on the number one cylinder as you continue to rotate the engine.

If its the exhaust valve, then you were at tdc, if its the intake valve, almost right away, you were not.

Rich

Hey George ,
I got nothing except maybe 1 dumb question ,do you got the spring in there between the distributor and the drive gear ( it keeps the drive gear from twisting
up&out and changing the timing ) maybe I miss that part of this post ,
sorry other then that I cant think of anything , watching that video ,it seems to
appear theres somthing to this timing Ignition thing , Good Luck Bill D.

does your distributor have a notch in the top edge of the body? the positioning of the #1 cap terminal pictured doesn't seem as i remember.

Rich, I'll give it a shot, but again, I don't see how my timing marks (this is the correct fan and it is bolted on properly) could point to TDC and the rockers be loose on that cylinder if it was not truly at TDC. I guess as you mentioned, it is theoretically possible that the engine was not assembled correctly and the valve gear is off. But this is a problem that seems to have developed over time - when I first got the car running, I never had a backfire. Which is *more* frustrating, given the decent compression and leakdown numbers. One would think that any problem that develops over time is a result of something mechanically wrong with the engine, or an incorrectly installed/broken part (read: dizzy, etc, but remember, I've replaced just about everything that isn't internal to the engine)

I do believe that you guys are on to something with the ignition timing, but I'm just having an issue conceptualizing why I would have a problem with one cylinder, but not others if it is indeed a timing problem.

Bill, that's a good point, but I do indeed have the spring installed in the drive gear.

rotor at notch on top distr body (if the body has one). 0* fan mark in the indicator notch. #1 wire terminal. doesn't need to be off by much to disrupt things.

Didn't have time to work on it last night, but I did some reading:
http://www.914world.com/bbs2/index.php?sho...08441&st=20

It might be possible that the distributor drive pinion (the offset fitting at the base of the dizzy shaft) is installed 180 degrees off. I pulled out the roll pin right after I got it so I could re-shim the pinion, and I guess it's theoretically possible that I put it back wrong.

It does not, however explain why I have the same problem with 3 different distributors though.

depends on the indexing and wire placement. if you are replacing the wires as before you're repeating the same behavior. the indexing being wrong is not so much in the distr as it is the dist driveshaft. the cog in the distr only fits one way into the shaft. look on pgs 64 and 65 for the text and fig 3.6 for the drive orientation @TDC.

Good call Kevin. In my mind, there was only one way to install the "cog", but since I don't have one in front of me, I was spitballing what was in my head.

So glad you made it to the WCR man! All that hard work paid off.

no hard work involved driving a car with 235hp, ac, partial leather seats, and sat radio.

k

So,

You went off and bought a total chick car, and now you just stopped working on the 914?

richard, the "bought a total chick car" was directed towards me or george? i just want to be absolutely sure before i reply any further.

I would assume its aimed at the miata that George bought.
Will be interesting to see if the msd changes the timing problem

It was directed at me . FYI, I haven't gotten to enjoy working much on either the 914 or the car. I have no garage down here in Florida, and it has rained throughout the day for the past, what seems like, 2 weeks. So during the summer my choices are: work out in the sun, or work in the rain. But thanks for the Rich, you're right, I need to get moving on it.

My next goal is to finish wiring the MSD as well as cutoff switch, since I know that currently the engine at least starts and runs fine most of the time. I'd like to get my wiring all set and reliable, then I'll swap out for this spare engine. So I'm not adding more unknown variables.

Didn't get to the MSD today, but I made some progress on my cutoff switch/ignition switch/push-button start. Not as much as I had hoped, as it seemed like everytime I went back outside, it would start to rain again. I feel like Charlie Brown with a cloud always following me around.

Wiring confuses the heck out of me. It's both an art and a science that I'm really not cut out for.

Everytime I turned around it was Raining - I must have gotten at least 3" at my House and it's starting again

ITs not art nor science, its wiring which is very logical.
Once you understand how a relay, coil, condenser and points work...the rest will fall right into place for you.

And its raining again.
Seems the east coast is getting hit with quite a bit of rain this year....and we need it, so w cant really complain to much.
But, I do have a place to work indoors...


Rich

So, how's that miata running?

All that rain has been my fault. I've been rain dancing non-stop to help those 12' trees take root.

I am still, however, waiting for George to figure this out.

I runs quite well Rich. And I even took the top down the other day to let the wind blow through my golden locks, just to piss you off.

Still working on wiring for now. Got a little work done yesterday and was GOING to work on it today till Dave must have started dancing. Hopefully sunshine the next few days, since my fiancee will be gone to the Keys for her bachelorette party. I have the driveway all to myself.

The plan is to finish the wiring in steps so I can check that I didn't screw anything up along the way. If the engine starts up again and nothing catches fire, then I'll start working on dropping it for the spare.

Hey Dave, if you're having any real dry spells, I learned a trick for planting trees and keeping them nice and watered at the roots. You can take 2 liter bottles and attach PVC at the end and push them into the ground till you get to the root ball/zone. Works like those little glass watering balls for household plants. Usually they'll last 3-4 days before needing to be refilled. You can also add a mixture of starter fertilizer that's high in phosphorous (20-20-20 works well) with the water to get them going.

Just don't let people see you with split ends....bad form.

Good luck with the wiring and I for one would like to do a triage on the motor to see why you think it may be the engine at this point.

If and when you take it apart, check the position of the timing gear in relationship to the crank gear and see if the dimple marks line up properly...before you pull the camshaft out of position.

Rich

Here's what I use

Click to view attachment