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Old 06-13-2016, 07:31 PM   #1
steward614
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Default Zenith rebuilt. No power past 40mph

Greetings. I am a jewelry who used my ultrasonic and steam generator to clean and inspect (10x magnification) a few old zenith carbs, pieces and parts to make two good units. The two run really well, new gaskets, i set the float but i did not perform a flow test for lack of equipment. Of the two carbs one has little power beyond 40mph, when i accelerate i hear the vacuum but just doesnt have the oomph. Might there be a chance that i do not have enough volume of fuel in the bowel? I shrived to adjust to 5/8" fuel level from the top however that took two gaskets to set the float needle to get the adjustment. No popping or backfires. So i dont believe its a rich flow issue. Any advice?
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Old 06-13-2016, 07:42 PM   #2
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Default Re: Zenith rebuilt. No power past 40mph

Does opening or closing the GAV at speed make any difference in engine performance?
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Old 06-13-2016, 07:46 PM   #3
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Cool Re: Zenith rebuilt. No power past 40mph

Use this site as a step by step guide to reassembly and operation

http://www.modela.org/main_jet.html

and buy a set flow tested jets and a fuel level gauge from renners

http://www.rennerscorner.com/index.html

Make sure your throttle shaft has 0 play or it will suck air... If so install an oversize shaft, most carbs need it
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Old 06-13-2016, 08:13 PM   #4
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Default Re: Zenith rebuilt. No power past 40mph

Utopia- I did turn the GAV open up-to two turns then back down to damn near off. Not a substantial change. All while running 40mph made an impact when open, some popping.
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Old 06-13-2016, 08:18 PM   #5
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Default Re: Zenith rebuilt. No power past 40mph

Mitch- I read the attached link to the main jet artificial on rebuilding. Appears to address my issues with high speed power. Without flow testing I am left with purchasing calibrated jets. I didnt know if there was an additional testing method while performing under load.
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Old 06-13-2016, 08:18 PM   #6
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Default Re: Zenith rebuilt. No power past 40mph

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Utopia- I did turn the GAV open up-to two turns then back down to damn near off. Not a substantial change. All while running 40mph made an impact when open, some popping.
Sounds like late timing.
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Old 06-13-2016, 08:26 PM   #7
Ron W
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Default Re: Zenith rebuilt. No power past 40mph

I agree with George. Double check your timing before buying new parts. Ron W
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Old 06-13-2016, 08:26 PM   #8
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Default Re: Zenith rebuilt. No power past 40mph

Mr Miller, good point. I recently replaced dist., points and cap and button. All adjusted to spec along with TDC of #1 on contact. Let me let her cool off tonight and Ill check the timing.
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Old 06-13-2016, 08:42 PM   #9
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Default Re: Zenith rebuilt. No power past 40mph

Did this engine run OK before? Ron W
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Old 06-13-2016, 08:42 PM   #10
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Default Re: Zenith rebuilt. No power past 40mph

Check your point gap and ensure it is within the specified 0.018-0.022" range. Going outside of that range affects timing and results in poor performance. Make sure the indent on your timing gear is centered exactly in the timing pin hole, too. (I know you asked about Zenith carbs, but like the guys say make sure your timing is spot on.)
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Old 06-13-2016, 09:26 PM   #11
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Default Re: Zenith rebuilt. No power past 40mph

If I understand you correctly, you have two carbs; one runs great, the other runs great below 40, but gives you trouble above 40. If that is correct, it would suggest that the problem is not ignition. If is was, you would experience the problem with both carbs.

I would agree that the likely culprit is a problem jet. I would suggest this... swap out the jets from the 'good' carb to the 'bad' carb. If that solves the problem, it would confirm that it is one or more bad jets.

If you do buy new jets, I agree with Mitch, get the ones from Renner.

Good luck,

Ken
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Old 06-13-2016, 09:46 PM   #12
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Default Re: Zenith rebuilt. No power past 40mph

George, you stole my line......
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Old 06-13-2016, 09:57 PM   #13
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Default Re: Zenith rebuilt. No power past 40mph

Quote:
Originally Posted by forever4 View Post
How did you clean the passages in the lower casting? Ultrasonic alone will not be reliable. I use electrolytic derusting, a Branson ultrasonic tank, and mechanical tools.

Two float valve gaskets is one too many. Your fuel level will be low. Did you remove the secondary well when you cleaned the casting?

The problem with many carbs is that they are not nearly as clean as they look, or as people think. I just did 8 more Zeniths on Sunday, and it took an entire afternoon to clean them all properly.
If you had that new GAV socket you could have done the job in half the time.
I'm still waiting on mine also.
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Old 06-13-2016, 10:31 PM   #14
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Default Re: Zenith rebuilt. No power past 40mph

I believe this is your #1 problem with that carb, " ...i did not perform a flow test..."

Here is a checklist to follow when rebuilding carbs,
Attached Files
File Type: pdf rebuilding carb worksheet.pdf (210.1 KB, 67 views)
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Old 06-14-2016, 03:05 AM   #15
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Default Re: Zenith rebuilt. No power past 40mph

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Actually, I just encountered the very first 7/16 hex GAV which I believe is original, not an old repro. It was on a Zenith 1 casting. The entire rest of the carb was an unmolested original, no signs of any tampering or previous disassembly.
Yep, I had one or two of those about 10 years ago, and also thought it was original.
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Old 06-14-2016, 05:06 AM   #16
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Default Re: Zenith rebuilt. No power past 40mph

Quote:
Originally Posted by KGBnut View Post
If I understand you correctly, you have two carbs; one runs great, the other runs great below 40, but gives you trouble above 40. If that is correct, it would suggest that the problem is not ignition. If is was, you would experience the problem with both carbs.

I would agree that the likely culprit is a problem jet. I would suggest this... swap out the jets from the 'good' carb to the 'bad' carb. If that solves the problem, it would confirm that it is one or more bad jets.

If you do buy new jets, I agree with Mitch, get the ones from Renner.

Good luck,

Ken

That's how I read it also....
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Old 06-14-2016, 05:27 AM   #17
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Default Re: Zenith rebuilt. No power past 40mph

The rennerscorner website doesn't appear to be working. Has anyone contacted them recently?
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Old 06-14-2016, 05:39 AM   #18
Mitch//pa
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Default Re: Zenith rebuilt. No power past 40mph

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The rennerscorner website doesn't appear to be working. Has anyone contacted them recently?
Bruce it's working for me
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Old 06-14-2016, 08:16 AM   #19
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Default Re: Zenith rebuilt. No power past 40mph

Tried it again and got in, but then no luck when I tried a different page. I'll keep at it.
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Old 06-14-2016, 09:25 AM   #20
steward614
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Default Re: Zenith rebuilt. No power past 40mph

Quote:
Originally Posted by forever4 View Post
How did you clean the passages in the lower casting? Ultrasonic alone will not be reliable. I use electrolytic derusting, a Branson ultrasonic tank, and mechanical tools.

Two float valve gaskets is one too many. Your fuel level will be low. Did you remove the secondary well when you cleaned the casting?

The problem with many carbs is that they are not nearly as clean as they look, or as people think. I just did 8 more Zeniths on Sunday, and it took an entire afternoon to clean them all properly.

Morning-I used the heated ultrasonic and each passage was blown out with 70psi of steam (a steam boiler we use in the Jewelry industry which has a fine tip outlet) and yes all jets were removed, cleaned and inspected. As for the float valve gaskets_ i added the second one to get the proper height of fuel in the bowl when the valve shuts off. However there may not be enough fuel once it gets both jets flowing at high speed...
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Old 06-14-2016, 09:36 AM   #21
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Default Re: Zenith rebuilt. No power past 40mph

Also I noticed there is a small bit of play in the throttle shaft. However does not effect idle so I didnt think it would impact high end speed performance.
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Old 06-14-2016, 10:10 AM   #22
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Default Re: Zenith rebuilt. No power past 40mph

I can tell you that there is more to a Zenith carb than you probably think. I found out the hard way. Save time and frustration and send the carb to Renner's Corner. No I do not have ties with them but the B carb on the A touring runs great.
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Old 06-14-2016, 01:14 PM   #23
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Default Re: Zenith rebuilt. No power past 40mph

A friend had the exact same problem with a brand new Zenith of recent production. A Zenith guru checked it, found the opening in the secondary well was just a bit below specs, enlarged it slightly to bring it up to specs, that was the problem. The jets were all Ok.
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Old 06-14-2016, 03:26 PM   #24
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Default Re: Zenith rebuilt. No power past 40mph

Quote:
Originally Posted by steward614 View Post
Also I noticed there is a small bit of play in the throttle shaft. However does not effect idle so I didnt think it would impact high end speed performance.
If there is play in the shaft air will get by it. Since your's is very little you may not notice the subtle differences but there are some occurring basically when the plate is closed. Other than running erratically leaner at idle I have had the rpm's stick a little higher from these shafts being worn. Going under the hood and manually pushing it against the throttle stop screw lowers it.. This is with no linkage binding and a good return spring on the accel bracket assy. All it takes is to open the hole up with a drill bit to install the oversize shaft.. Very easy fix...
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