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1275 backfiring

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Atrus

Atrus

Jedi Warrior
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Hey All,

I have a '68 Sprite 1275cc. I've been battling trying to get the timing/carbs set right (I'm 25 and grew up in the age of fuel injection /ubbthreads/images/graemlins/wink.gif )

Anyway, I made sure #1 was a TDC, and looked at the timing mark on the crank pulley - it was in the right position relative to the timing guide. I started the car up, got it to 10* BTDC...car actually seems to be idling smoothly, considering that I know the carbs are still way out of whack (had to be as I had the timing really off before). The issue I have now is the car seems to be backfiring through the carbs, spraying raw gas out the carbs. I would think the #1 culprit of this would be timing, which is why I made sure #1 was a t TDC and the mark was positioned corrrect, incase the engine was ever rebuilt and not aligned properly. When I severely retard the timing (about 40*), I get no backfire, this is where I had tuned it "by ear" prior to setting it with the gu today. Explains why it would never run correctly :O

One thought I do have is valve springs - do you think the springs could be weak, causing an intake valve to remain open? I don't know the milage or history of the car. When I got it, however, I do know that the carbs were shot (pistons seemed scored and warped on them) and off the car had sat for about 10 years. That indicates to me that a couple of valve springs were compressed for 10 years without moving.

Any advice/experience/tips would be appreciated. I have my registration coming hopefully this week so I can get this LBC on the road! I'm anxious!
 
Is the timeing correct with the engine running?

Wonder if you have a street cam? Seem to remember my street cam liked about 30 degrees. Set the timeing by ear. Advance it some and try that, if better advance some more. Really hard to tell till you get it on the road and can test it under load. But should not spit back in any case.
 
Sorry, should have been clearer - I set the timing with the engine running using a timing gun. So, that's correct. I suppose it could have a cam in it...issue is, if I advance it any more, the backfiring gets worse. If I retard the timing quite a bit, the backfiring stops. When I timed it by ear, I was about 60* retarded on the timing.
 
First off, Welcome.

Explain to us how you are timing it and where you get the 60' retarded timing figure. I don't think my car will run at 60' retarded. Is the vacuum advance vacuum line connected and plugged? What is the engine RPM when timing? Are you connected to the plug wire for the farthest front cylinder?
 
Do you supose, just supose, some one put it together 180 out? Or of course the Dizzy cam gear is off a tooth.
 
The dizzy position shouldn't matter for the timing figures; that just affects where the body and cap are in relation to the block (which is a problem as far as clearance to rotate it with the vacuum advance sticking out the side). If he is taking the timing marks from the crank pulley with the pickup off of #1 cylinder and really seeing 60' BTDC then the only things I can think of are that the cam isn't timed right (but I assume the engine ran fine at some point – please clarify the engines history) or the rubber on the damper is sheared and the outside of the damper has moved in relation to the inside. I guess what I'd like to see is the valve positions for #1 cylinder when the crank pulley is lined up at TDC and the dizzy rotor is pointing to the #1 plug wire, or vice versa what the timing mark is when #1 is known to be at TDC before the power stroke, but this is a little harder to tell with the head on (it can be done though).
 
Well ok, remove the valve cover and note when both valves on #4 are open and both valves on #1 are closed. Rotate the engine a bit further till the notch in the pully lines up with the right pointer on the cam cover. Now check that the Dizzy roter is pointing at #1 wire on the cap.

And that should be preaty darn close.

Oh yea, valves down is open, up is closed.

What did I forget Chris?
 
[ QUOTE ]
Well ok, remove the valve cover and note when both valves on #4 are open and both valves on #1 are closed. Rotate the engine a bit further till the notch in the pully lines up with the right pointer on the cam cover. Now check that the Dizzy roter is pointing at #1 wire on the cap.

And that should be preaty darn close.

Oh yea, valves down is open, up is closed.

What did I forget Chris?

[/ QUOTE ]

Thanks everyone, that's exactly what I did. I think I have it set, I got the timing at 10* BTDC when I messed with the carbs (they were FAR too rich), and it isn't backfiring. My issue now is trying to sync up the carbs. The car is running fast, it's at a good 700-1,000 rpm idle speed, however, both of the throttle adjustment screws are backed off fully. It seems that the carb nearest the front of the engine is doing all the work, the carb near the rear is hardly doing anything at all. If I increase the rearward carb's idle speed, the car will be idling too high with no way to back it down, as the screws are completely backed off.

For clarity, I am sure the car is at 10* btdc because I pulled the valve cover to ensure, indeed, that both #1 cylinder valves were closed on the compression stroke of #1. I suppose like someone mentioned, the cam could be off.

I don't know the history at all - I got the car with the carbs disassembled and pulled off. I don't know the milage or the # of rebuilds, if any, it's had. It had sat for about 10 years before I picked it up with underwear (ew) plugging the intake manifold and the carbs in a box. I bought 2 new HS2 carbs from Moss Motors, the piston in the old carbs was scored up and sticking pretty badly.
 
Hate to ask but feel I must. You are adjusting idle speed with the carbs disconnected from each other. Idle is important but so is balance. Listen closely, if your ears are good you can do it. However, lots of tools some homemade and some expensive for this job. Get em balanced about 700 to 800 RPM then lockem together.

Wow, new carbs, you do know the old ones can be professionaly rebuilt for $350 and polishing included. Now you know, sigh. Hay this is the place to ask first.
 
So new carbs from Moss?

Okay, then we can rule out worn throttle spindles/bushings.

As Jack said, set the carbs to have equal air flow by ear. You can use a piece of 3/8" fuel line up to your ear and the carb intake to help you figure this out. Don't worry about the idle speed yet, just get to sound the same. After that, use the combined throttle adjustment to drop the idle. If you have to, you can revisit each carb and drop their idle down lower, but always keep them ballanced.

To determine air/fuel mixture, do the following:
At idle (whatever that may be for now), lift a piston 1/16". If you have a little piston plunger on the side of the carb, use it.

- if the RPM drops, you're running lean.
- if the RPM rises and stays high, you're running rich
- if the RPM stays the same or rises and falls back again, you're pretty close.
 
Thanks guys...yeah, I bought the HS2's off of Moss a few years ago (had the car for 4 years now, finally out of college and ready to use it!)....anyway, I think they were only about $350-$400 on a sale, so I figured I'd just go new.

I have the choke and throttle cable disconnected so each carb is independant of the other. The issue I have is just that both of the throttle screws are backed off, and the car is idling at about 800 rpm. Then, in order to balance the carbs (I have a sync tool I got from the previous owner), I'd have to increase the rearward carb, which is going to make the idle too high. I'd have no way to bring it back down. I have stock needles and jets in the carbs, and I turned the jets up to the flush with the bridge, then backed off 2 full turns per the repair manual. It just seems that the front carb is working too much, with no way to bring the idle down as the throttle screws are completely backed off on both carbs.

Any advice on why it's idling to high?
 
Do the dash pot pistons, needles fall all the way into the seats, should shut off the fuel. Are the needles mounted correctly in the dash pots?

Seperate carbs.
Unscrew the idle adjustment so the butterfly is closed, look in there.
Disconnect choke cable.
Set bottom of mix jet to level with the bridge. Open them two turns(12 flats), screw down toward ground to open.
Start engine.
Adjust idle so both are the same.
Adjust mix nut till you get fastest idle, readjust idle screw as needed.
Rinse and repeat untill you got it right.
Now do the rear carb the same way.

To check work raise the piston about 1/32 of an inch. Small screw driver works well.
If speed increases Mix to rich, if speed immidiately drops is too rich.
If increases a bit then stabilizes is good.
Hook up choke linkage.
Tie carbs bach together with the funny clamp thing.

Play a bit you'll get it.
 
Hey all....

Well, I just wanted to reply to this and say thanks for the tips/advice. I know the topic's gone offhand, from timing to carbs.

The update is, I think I just found the problem. I don't think I had the fast idle screw backed off enough from the front carb, so even though I thought the butterfly was closing, indeed, it was not. So, I am sitting here enjoying a Newcastle (MMMmmmm) in hopeful victory, and I'll be out to set the idle and mixture in just a minute. Chalk that one up to a lesson well learned.

Thanks again everyone! I am hoping to get her out tonite for a nice cruise, should be a great night for it here in MI!
 
Kevin!! Welcome back to the fold. The last I knew you were going to sell the Sprite.
I should have jumped in the Midget, and ran over to share a Newcastle with you.
Let me know where you are in Sterling Heights, and we can hook up.
Jeff
 
Thanks Jeff! Yeah, I actually have it for sale in the Imlay City paper (my wife works there), but I figure if I end up loving it, I'll keep it. I JUST took it for a 7 mile ride! :smile:

Since I was last posting on the board, I got married and bought a house at 18 and Van Dyke. Luckily, the G6, Prix, and Healey all fit in the detatched garage! :-D

So, I'm off to take a shower and head over to my friend's party in the Sprite!
 
Kevin, 18 and Van Dyke is only an hour from me. I used to work on 15 between Mound and Van Dyke, so know the route well. I'd just come across 59 to Van Dyke.
Congratulations on the marriage, and the house. (And the garage!!)
Keep the Sprite!
Jeff
 
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