How do I adjust the timing on a Nissan 25hp

[Papafrankc]

Does anyone know how to adjust the timing on a 2002 Nissan 25hp - 3 carb
4 cycle engine?

The service manual that I have talks about how to check the timing with a light. It also talks about the 10 or 11 settings you can use, 9 BTDC and 2 ATDC. I've used the light & I can see the current setting fine.

But there is no mention in the manual as to how to change the setting??

Any help will be appreciated.
Thanks,

 

[pvanv]

Re: How do I adjust the timing on a Nissan 25hp

Get us the precise model, as year of manufacture is not a good identifier on these. Then we can help you some more.

Since you have checked the timing, is it OK, or does it need adjustment? It's unusual for this to be off, unless some major work has been done to the motor recently.

Do you have a Factory service manual? That should help you set the timing; the aftermarket manuals aren't too good.

 

[Papafrankc]

Re: How do I adjust the timing on a Nissan 25hp

Paul,

Thanks for your reply. Here's the model nbr info:
Nissan 25A EPT
My owners manual says NSF 25A

As to your other questions:
Using my timing light I can see that the timing appears to be right at TDC when at idle.
I have the GREEN Service manual (93 pages) (OB NO. 003N21053-0-NS). As far as I know this is the factory service manual? And on pg 91 para 5) it tells how to check the timing and what it should look like but I don't see any info on how to adjust it if it is not correct?

Some comments and questions:
On page 1 it says for Ignition Timing - ATDC 2 deg - BTDC32 deg (Elect advance). After checking the timing chart on pg 91 I'm assuming that at an idle of about 1000 rpm it should be at ATDC 2 deg, then as the rpm rises to about 6000 the timing should advance to BTDC32.

I don't remember seeing it advance at all while increasing the rpm, but I'm not sure if I checked it WHILE advanced the rpm. I'll have to verify this point.

Here's some info on what I've done to the motor and why I'm thinking about the timing maybe being off:
Current problem:
Considerable lack of power at full speed.
Checking the spark plugs they are carbon fouled black. Put in new plugs, same black color after only 5-10 minutes.

I believe this problem existed just before I made the changes listed below. However that was some time ago so I'm not 100% sure? My last run before the changes was at an altitude of 6500 ft and it ran poorly. I assumed it was because of the altitude but now am not sure.

Here's what I have done to the engine (after that run at 6500ft):
1- I replaced the jets and other parts in all three carbs to increase the hp from 25 to 30hp.
2- The engine starts better than it did as a "25". It idles fine. I'm guessing that the engine "likes" the richer mixture. Just a guess as I said.
3- I've checked for clean fuel etc. and I don't believe I have a fuel problem.

Some of my thoughts:
I'm thinking that because of the black plugs that it's running too rich?
Advancing the timing should make the plugs run hotter, thus helping the carbon black problem?
Howver, the timing specs for the 25 and the 30 in the manual are the same so I'm guessing that the timing does not have to be changed for the 30?

After my recent research I'm wondering if the Electric Timing advance might not be working?

Next week, when I'm able to get to the engine, I'll check the timing to see if it advances when I rev up the engine. If it's not advancing, I guess it could explain the loss of power at full speed.

Sorry for the long winded reply, but I wanted to give you as much background as I can.

Thanks again for your help...

 

[pvanv]

Re: How do I adjust the timing on a Nissan 25hp

Frank,

I think you have the proper service manual.

Ugh. Yes, it sounds pretty rich. You may have actually caused that by changing to the bigger jets/nozzles. Different jets would be recommended for operation at 6500 feet. With the sea-level jets, you probably wouldn't get full power when you're more than a mile above sea level. So that may have been your high altitude running issue.

If the initial timing is correct, the rest of the advance should be automatic, controlled by the ECU (the CD box). You can't change the advance curve, but can only make sure that the sensors (pulser coils) are in the correct spot so the initial spark timing is good. As long as it's close, the ECU should take care of the rest. In other words, if it doesn't advance, it's time for a new ECU. Yes, more advance will generally result in hotter burn, but your symptoms sound like it's way too rich for any amount of advance to burn cleanly.

I think you will find that the venturi size is different between the 25 and 30 carbs, to allow for the increased jet size. In other words, the carb casting itself is bigger for the 30. If you just put in the bigger jets/nozzles, you have likely made the mixture way too rich, resulting in the sooting on the plugs. Of course, idle is the least affected; WOT is the most affected by jet changes. Hence your decent idle, but sooting above that. When we have changed from 25 to 30 HP, we have done whole carbs.

I think that you will find that if you reinstall the original 25 hp carb bits, the plugs will likely run clean again. While at it, verify the cam timing (usually OK, unless the gear belt jumped a tooth due to backfire), and adjust the valves. Do the valve adjustment after you have run the sooting out of the motor.

Then, if you REALLY want that additional 5 hp, you will need 3 new carbs.

 

[Papafrankc]

Re: How do I adjust the timing on a Nissan 25hp

Paul,

Thanks for the quick reply. I'll be checking the timing next week to see if it is off or OK.

About the possible carb venturi size. In the parts diag on page 10, there are 2 different listings(25hp & 30hp) for each of the carbs:
Ref No's

5-1 Upper carb
5-2 Center carb
5-3 Lower carb​

I believe these listings are for the WHOLE cabruretor.

Then the only other parts that are different for the 25 & 30 are the 1-main nozzle, 2-jets and the link rod. I don't see any listing for different body assemblies? However, it looks like if I wanted to buy just the body assembly for either the 25 or the 30, then I would have to buy the whole carb. Then if the venturi's are diffferent for the 2 models then the correct venturi size would automatically come with each carb.

By chance have you ever compared the venturi's from the 2 engines and seen if there is an actual difference? This possible difference could be a key factor in my do-it-yourself horsepower upgrade.

FYI, I'm back at sea level now so the high altitude won't be a problem. Just for my education...would I have to re-jet to smaller jets if I went back to high altitude again? I was never able to find any info about what size jets to use at a higher altitude.

When I get the timing verified I'll post the results.

Thanks again for your help.
Frank

 

[pvanv]

Re: How do I adjust the timing on a Nissan 25hp

I have not personally compared or measured the 25 and 30 carbs, so can't say specifically that the 30 is bigger. I do know that if you re-jet (only) you will change high speed mixture. I have verified that the venturi sizes do indeed differ for the 4/5/6 and the 8/9.8

Yes, high altitudes call for smaller jets. You can call the factory for specific recommendations for your motor depending on altitude. While you have them on the line, you can also see if they have 25 and 30 carbs handy, to look at the venturi sizing.