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z31sham

1970 240Z Timing/Ignition/Carb Question

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I'll stick this here knowing the subject has probably been beat to death already.

 

Recently rebuilt mikuni 44phh carbs and added a Schneider 270-80f cam to E88 head, 2.6l block. Fuel pressure is at 3.5psi.

 

The car started a little rough with a lot of backfire through the carb. I thought of the dizzy being 180 out but confirmed it wasn't.

 

I have start to play around with advancing the timing. 

 

Initially it was idling at 0deg and rough, 2-3000rpm would hit 15-18deg and smooth out the roughness. Fuel pressure is at 3.5psi.

 

I currently have the:

  • Mikuni pilot screws are all out 1.5 turns.
  • Idle sitting 1000rpm
  • 15 advance at idle 
  • 2-3000rpm advance is probably at 30deg (off the scale)
  • Still getting backfire through carbs particularly on cylinder 1 at idle.
Next steps tonight are too sync the carbs, adjust with the idle on backfiring carbs - am I missing anything? I've attached an idle video and picture of jet sizes.
 
Any input or comments would be helpful.
 

IMG 0691

IMG 0695

 

 

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Pop the top cover off the carb and make sure all the jets are tightened all the way down. I recently rebuilt my phh40s and the top plate of the carb didn't line up just perfectly. So the pilot jet bound up against the side of the top plate and didn't seat all the way down. Had similar results. If it's not that then check all jets.

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I don't know about 44s but on my 40s with flat tops and a saved p90 head, those 52 pilot jets would be way too lean. I recommend you do the following until you get a wideband AFR gauge and/or color tune tool:

 

1) check vacuum adv hose is connected to the barb located on carb #1

2) set your timing 12-15 deg at idle

3) disconnect your three linkage arms form the carbs, loosen the bolts on two arms until they are free to rotate on the shaft.

4) sync all carbs using a sync tool - same air flow

5) reconnect the one linkage arm that you didn't loosen, then connect the other two arms to the carbs and tighten the bolts to lock the arms to the shaft.

6) working one mixture screw at a time slowly turn each to achieve max rpms. As I said before I think you are lean so probably need to turn them out to begin with. more than two turns out then you should consider a larger pilot jet. But for now jest keep adjusting to get max rpms.

7) repeat step 6

8) repeat step 4 (Check it)

9) report back your results

 

one other thing to consider, and its huge to the proper function of your carbs, is the float levels. Did you set them per the spec? Even if you did set them hanging upside down like the spec says, that's no guarantee that the fuel levels will be the same correct levels. To check the fuel levels do this:

1) take top cover off carbs

2) remove one of the main/air jet tube assemblies out of the jet block

3) measure from top of jet block down to the fuel level on each carb. I use a Vernier caliper's projecting arm to do this.

4) if you have the external adjusters, adjust fuel level. I set mine at 22-23mm down from the jet block. Your 44s may vary.

Good luck!

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I have:

60 pilot winter / 57.5 summer

150 main winter / 145 summer

200 air

45 pump nozzles

15 deg adv at idle

 

Just watched your video again, the vacuum hose coming off the first carb, what is that connected to? I see you have it teed off. Recommend you disconnect that for now and just run a straight piece of hose from that barb on the first carb to the adv on the carb. Just to rule out vacuum leaks. You can reconnect it later after you get your idle sorted out.

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