Greetings
I’m currently working on a 92 Honda Accord which has been sitting for 5 years and wouldn’t run because of a clogged carb and missing vacuum lines.

The biggest issue I’ve been trying to solve is getting the engine to idle and rev smoothly with A/C on. The previous owner replaced the whole distributor with the ignition coil (OEM Hitachi) and I replaced the wires and spark plugs with stock NGKs. Ignition timing is on point and I check valve clearance. This is a carburetor model (very rare and limited info). I rebuilt it with a kit.

I also studied and simplified the vacuum system. Made absolute sure with a smoke machine that there are no vacuum leaks from bad diaphragm,etc.

After doing all this, the engine will not run smoothly with the AC on. The idle booster works and maintains 1k rpm but there is a misfire. The engine shakes because it is running on 3cyls. Checked by pulling each wire and it is not always the same cylinder that’s misfiring.

The only thing that seemed to fix it is by making the spark gap to 0.025” rather than specified 0.039”. I did this not expecting any difference but surprisingly there were. The engine idles and revs smoothly with no hesitation with or without AC. Drove it in traffic and not a single misfire unlike before.

I simply can’t figure out why the engine will not run right with the correct gap? I understand the whole “if it ain’t broke, don’t fix it” phrase since it runs perfectly now, but I’d like to understand why such a change helped.

I thought it could be a bad ground but cleaning the grounds and grounding the block with a jump start cable made no difference. Tested the coil and resistance are bang on as specified on the shop book.

Would greatly appreciate any thoughts.
Thank you

 

What I am about to say is not a true statement, but it also works in a weird way.

When i imagine a change in sp gap, I think of it in terms of the gap acting like it is changing the timing in an odd way. I had always set mine to .034-.035 as I felt .038-.039 was too wide of a gap.

For example, most hotrodders will say to widen the gap and gain more power due to a cleaner burn or what ever.
I find that with my Honda's 86-94 that my cars run rough, less power and worse fuel mileage when the plugs are gapped too wide. (from use/miles). When I re-gap my used plugs, to stock the car runs even better.

I am curious to try this on my own cars this spring/summer to see if I notice a difference.

After I set my timing, I also advance the dist slightly. This gains me smoother acceleration, better fuel mileage and changes my HP/Torque curve (power band) to a slightly higher rpm (not in actuality, but in how the performance feels). My main driving is done at either just over 70mph or just over 80mph here in Montana. Living in a rural area and mainly use a highway or interstate. A consistent 33.5mpg with only highway driving at 70+mph with a 5 speed and over 300k miles on my car at 5k-6,500k elevation on cheap fuel.

I willkeep thinking on why the car seems like it is running on 3cyl with a proper gap.

 

I wonder if the timing belt is off 1 tooth?

 

Thank you Micro for the response. I am curious to see how the engine will perform with the timing slightly advanced. Fuel consumption is not great but I can’t expect that with a 2L carb engine :/

Additionally, I hooked up my trusty vacuum gauge and the needle reads steady at 21. That tells me the engine is in good health.


As for the timing belt being off by 1 tooth, that gave me shivers haha. The timing belt, water pump,etc were replaced by the previous owner (being my grandfather) by the dealer.
I need to remove the valve cover to replace the gasket so I will check if all the marks line up.

Am I correct to say that if the camshaft “TOP” and flywheel TDC mark line up, everything is in order?

 

I forget exactly with the cam gear if it said top or not, but there should be a line on the left and right of the cam gear that align with the top where the valve cover and head meet. Remember the head is slightly tilted toward the front of the car. It should be very straight forward.

Also, if you have an automatic transmission. The 90-93 Accords got 4 or so less mpg vs the stick.

 

I just finished checking valve clearances and changed the cover gasket.

Interestingly Cylinder 2 and 3 Exhaust valves were tighter than specified, 0.010” slip through with too much resistance. Corrected them to 0.012” as specified in the manual. Intake clearances were all within spec at 0.010”.

I’m wondering if this could be the cause of my issues. The spark gap is still same as before (less than specified). Advanced the timing a tad bit. Engine idles smoothly like before.

One question, is it normal for the vacuum gauge to decrease to less than 18 with AC and Headlights on? I presume this is because of load on the engine?

I verified the timing marks on the camshaft and flywheel line up correctly.

(The car is a manual carburetor version, therefore the timing marks are on the flywheel, not the crank pulley)

 

>002 is not enough to of a valve clearance issue to make any difference in how it runs. I would not really be concerned until you hit about .005 - .007 or more.
I have never had need to check vacuum on a Honda, but if it drops vacuum on an alternator load, I would assume that this is from the lower rpm caused by the alternator as well.